In pointing out in a former chapter the evils which have resulted from

the too extensive destruction of the forests, I dwelt at some length on

the increased violence of river inundations, and especially on the

devastations of torrents, in countries improvidently deprived of their

woods, and I spoke of the replanting of the forests as probably the most

effectual method of preventing the frequent recurrence of disastrous

floods. There are many regions where, from the loss of the superficial

soil, from financial considerations, and from other special causes, the

general restoration of the woods is not, under present circumstances,

either possible or desirable. In all inhabited countries, the

necessities of agriculture and other considerations of human convenience

will always require the occupation of much the largest proportion of the

surface for purposes inconsistent with the growth of extensive forests.

Even where large plantations are possible and in actual process of

execution, many years must elapse before the action of the destructive

causes in question can be arrested or perhaps even sensibly mitigated by

their influence; and besides, floods will always occur in years of

excessive precipitation, whether the surface of the soil be generally

cleared or generally wooded. [Footnote: All the arrangements of rural

husbandry, and we might say of civilised occupancy of the earth, are

such as necessarily to increase the danger and the range of floods by

promoting the rapid discharge of the waters of precipitation.

Superficial, if not subterranean, drainage is a necessary condition of

all agriculture. There is no field which has not some artificial

disposition for this purpose, and even the furrows of ploughed land, if

the surface is inclined, and especially when it if frozen, serve rather

to carry off than to retain water. As Bacquerel has observed, common

road and railway ditches are among the most efficient conduits for the

discharge of surface-water which man has yet constructed, and of course

they are powerful agents in causing river inundations. All these

channels are, indeed, necessary for the convenience of man, but this

convenience, like every other interference with the order of nature,

must often be purchased at a heavy cost.] Physical improvement in this

respect, then, cannot be confined to merely preventive measures, but, in

countries subject to damage by inundation, means must be contrived to

obviate dangers and diminish injuries to which human life and all the

works of human industry will occasionally be exposed, in spite of every

effort to lessen the frequency of their recurrence by acting directly on

the causes that produce them. As every civilized country is, in some

degree, subject to inundation by the overflow of rivers, the evil is a

familiar one, and needs no general description. In discussing this

branch of the subject, therefore, I may confine myself chiefly to the

means that have been or may be employed to resist the force and limit

the ravages of floods, which, left wholly unrestrained, would not only

inflict immense injury upon the material interests of man, but produce

geographical revolutions of no little magnitude.

Inundations of 1856 in France.

The month of May, 1856, was remarkable for violent and almost

uninterrupted rains, and most of the river-basins of France were

inundated to an extraordinary height. In the val-leys of the Loire and

its aflluents, about a million of acres, including many towns and

villages, were laid under water, and the amount of pecuniary damage was

almost incalculable. [Footnote: Champion, Les Inondations en France,

iii., p.156, note.] The flood was not less destructive in the valley of

the Rhone, and in fact an invasion by a hostile army could hardly have

been more disastrous to the inhabitants of the plains than was this

terrible deluge. There had been a flood of this latter river in the year

1840, which, for height and quantity of water, was almost as remarkable

as that of 1856, but it took place in the month of November, when the

crops had all been harvested, and the injury inflicted by it upon

agriculturists was, therefore, of a character to be less severely and

less immediately felt than the consequences of the inundation of 1856.

[Footnote: Notwithstanding this favorable circumstance, the damage done

by the inundation of 1840 in the valley of the Rhone was estimated at

seventy-two millions of francs.--Champion, Les Inondations en France,

iv., p. 124.

Several smaller floods of the Rhone, experienced at a somewhat earlier

season of the year in 1846, occasioned a loss of forty-five millions of

francs. "What if," says Dumont, "instead of happening in October, that

is, between harvest and seedtime, they had occurred before the crops

were secured The damage would have been counted by hundreds of

millions."--Des Travaux Publics, p. 99, note.]

In the fifteen years between these two great floods, the population and

the rural improvements of the river valleys had much increased, common

roads, bridges, and railways had been multiplied and extended, telegraph

lines had been constructed, all of which shared in the general ruin, and

hence greater and more diversified interests were affected by the

catastrophe of 1856 than by any former like calamity. The great flood of

1840 had excited the attention and roused the sympathies of the French

people, and the subject was invested with new interest by the still more

formidable character of the inundations of 1856. It was felt that these

scourges had ceased to be a matter of merely local concern, for,

although they bore most heavily on those whose homes and fields were

situated within the immediate reach of the swelling waters, yet they

frequently destroyed harvests valuable enough to be a matter of national

interest, endangered the personal security of the population of

important political centres, interrupted communication for days and even

weeks together on great lines of traffic and travel--thus severing, as

it were, all South-western France from the rest of the empire--and

finally threatened to produce great and permanent geographical changes.

The well-being of the whole commonwealth was seen to be involved in

preventing the recurrence and in limiting the range of such

devastations. The Government encouraged scientific investigation of the

phenomena and their laws. Their causes, their history, their immediate

and remote consequences, and the possible safeguards to be employed

against them, have been carefully studied by the most eminent

physicists, as well as by the ablest theoretical and practical engineers

of France. Many hitherto unobserved facts have been collected, many new

hypotheses suggested, and many plans, more or less original in

character, have been devised for combating the evil; but thus far, the

most competent judges are not well agreed as to the mode, or even the

possibility, of applying an effectual remedy. I have noticed in the next

preceding chapter the recent legislation of France upon the preservation

and restoration of the forests, with reference to their utility in

subduing torrents and lessening the frequency and diminishing the

violence of river inundations. The provisions of those laws are

preventive rather than remedial, but most beneficial effects have

already been experienced from the measures adopted in pursuance of them,

though sufficient time has not yet elapsed for the complete execution of

the greater operations of the system.

Basins of Reception.

Destructive inundations of large rivers are seldom, if ever, produced by

precipitation within the limits of the principal valley, but almost

uniformly by sudden thaws or excessive rains on the mountain ranges

where the tributaries take their rise. It is therefore plain that any

measures which shall check the flow of surface-waters into the channels

of the affluents, or which shall retard the delivery of such waters into

the principal stream by its tributaries, will diminish in the same

proportion the dangers and the evils of inundation by great rivers. The

retention of the surface-waters upon or in the soil can hardly be

accomplished except by the methods already mentioned, replanting of

forests, and furrowing or terracing. The current of mountain streams can

be checked by various methods, among which the most familiar and obvious

is the erection of barriers or dams across their channels, at points

convenient for forming reservoirs large enough to retain the superfluous

waters of great rains and thaws. [Footnote: On the construction of

temporary and more permanent barriera to the curreuts of torrents and

rivulets, see Marchand, Les Torrents des Alpes, in Recue des Eaux et

Forets for October and November, 1871.]

Besides the utility of such basins in preventing floods, the

construction of them is recommended by very strong considerations, such

as the furnishing of a constant supply of water for agricultural and

mechanical purposes, and, also, their value as ponds for breeding and

rearing fish, and, perhaps, for cultivating aquatic vegetables.

[Footnote: In reference to the utilization of artificial as well as

natural reservoirs, see Ackerhof, Die Nutruny der Teiche und Gewasser,

Quadlinburg, 1869.]

The objections to the general adoption of the system of reservoirs are

these: the expense of their construction and maintenance; the reduction

of cultivable area by the amount of surface they must cover; the

interruption they would occasion to free communication; the probability

that they would soon be filled up with sediment, and the obvious fact

that when full of earth, or even water, they would no longer serve their

principal purpose; the great danger to which they would expose the

country below them in case of the bursting of their barriers; [Footnote:

For accounts of damage from the bursting of reservoirs, see Vallee,

Memoire sur les Reservoir d'Alimentation des Canaux, Annales des Ponts

et Chaussees, 1833, 1er semestre, p.261.

The dam of the reservoir of Puentes in Spain, which was one hundred and

sixty feet high, after having discharged its functions for eleven years,

burst, in 1802, in consequence of a defect in its foundations, and the

eruption of the water destroyed or seriously injured eight hundred

houses, and produced damage to the amount of more than a million

dollars.--Aynard, Irrigations du Midi d l'Europe, pp. 257-259.] the evil

consequences they would occasion by prolonging the flow of inundations

in proportion as they diminished their height; the injurious effects it

is supposed they would produce upon the salubrity of the neighbouring

districts; and, lastly, the alleged impossibility of constructing

artificial basins sufficient in capacity to prevent, or in any

considerable measure to mitigate, the evils they are intended to guard

against.

The last argument is more easily reduced to a numerical question than

the others. The mean and extreme annual precipitation of all the basins

where the construction of such works would be seriously proposed is

already approximately known by meteorological tables, and the quantity

of water, delivered by the greatest floods which have occurred within

the memory of man, may be roughly estimated from their visible traces.

From these elements, or from meteorological records, the capacity of the

necessary reservoirs can be calculated. Let us take the case of the

Ardeche. In the inundation of 1857, that river poured into the Rhone

1,305,000,000 cubic yards of water in three days. If we suppose that

half this quantity might have been suffered to flow down its channel

without inconvenience, we shall have about 650,000,000 cubic yards to

provide for by reservoirs. The Ardeche and its principal affluent, the

Chassezae, have, together, about twelve considerable tributaries rising

near the crest of the mountains which bound the basin. If reservoirs of

equal capacity were constructed upon all of them, each reservoir must be

able to contain 54,000,000 cubic yards, or, in other words, must be

equal to a lake 3,000 yards long, 1,000 yards wide, and 18 yards deep,

and besides, in order to render any effectual service, the reservoirs

must all have been empty at the commencement of the rains which produced

the inundation.

Thus far I have supposed the swelling of the waters to be uniform

throughout the whole basin; but such was by no means the fact in the

inundation of 1857, for the rise of the Chassezae, which is as large as

the Ardeche proper, did not exceed the limits of ordinary floods, and

the dangerous excess came solely from the headwaters of the latter

stream. Hence reservoirs of double the capacity I have supposed would

have been necessary upon the tributaries of that river, to prevent the

injurious effects of the inundation. It is evident that the construction

of reservoirs of such magnitude for such a purpose is financially, if

not physically, impracticable, and when we take into account a point I

have just suggested, namely, that the reservoirs must be empty at all

times of apprehended flood, and, of course, their utility limited almost

solely to the single object of preventing inundations, the total

inapplicability of such a measure in this particular case becomes still

more glaringly manifest.

Another not less conclusive fact is, that the valleys of all the upland

tributaries of the Ardeche descend so rapidly, and have so little

lateral expansion, as to render the construction of capacious reservoirs

in them quite impracticable. Indeed, engineers have found but two points

in the whole basin suitable for that purpose, and the reservoirs

admissible at these would have only a joint capacity of about 70,000,000

cubic yards, or less than one-ninth part of what I suppose to be

required. The case of the Ardeche is no doubt an extreme one, both in

the topographical character of its basin and in its exposure to

excessive rains; but all destructive inundations are, in a certain

sense, extreme cases also, and this of the Ardeche serves to show that

the construction of reservoirs is not by any means to be regarded as a

universal panacea against floods.

Nor, on the other hand, is this measure to be summarily rejected. Nature

has adopted it on a great scale, on both flanks of the Alps, and on a

smaller, on those of the Adirondacks and of many lower chains. The

quantity of water which, in great rains or sudden thaws, rushes down the

steep declivities of the Alps, is so vast that the channels of the Swiss

and Italian rivers would be totally incompetent to carry it off as

rapidly as it would pour into them, were it not absorbed by the

capacious basins which nature has scooped out for its reception, freed

from the transported material which adds immensely both to the volume

and to the force of its current, and then, after some reduction by

evaporation and infiltration, gradually discharged into the beds of the

rivers. In the inundation of 1829 the water discharged into Lake Como

from the 15th to the 20th of September amounted to 2,600 cubic yards the

second, while the outflow from the lake during the same period was only

at the rate of about 1,050 cubic yards to the second. In those five

days, then, the lake accumulated 670,000,000 cubic yards of superfluous

water, and of course diminished by so much the quantity to be disposed

of by the Po. [Footnote: Baird Smith, Italian Irrigation, i., p. 176.]

In the flood of October, 1868, the surface of Lago Maggiore was raised

twenty-five feet above low-water mark in the course of a few hours.

[Footnote: Bollettino della Societa Geog. Italiana, iii., p. 466.] There

can be no doubt that without such detention of water by the Lakes Como,

Maggiore, Garda, and other subalpine basins, almost the whole of

Lombardy would have been irrecoverably desolated, or rather, its great

plain would never have become anything but a vast expanse of river-beds

and marshes; for the annual floods would always have prevented the

possibility of its improvement by man. [Footnote: See, as to the

probable effects of certain proposed hydraulic works at the outlet of

Lake Maggiore on the action of the lake as a regulating reservoir,

Tagliasecchi, Notizie sui Canali dell' Alta Lombardia, Milano, 1869.]

Lake Bourget in Savoy, once much more extensive than it is at present,

served, and indeed still serves, a similar purpose in the economy of

nature. In a flood of the Rhone, in 1863, this lake received from the

overflow of that river, which does not pass through it, 72,000,000 cubic

yards of water, and of course moderated, to that extent, the effects of

the inundation below. [Footnote: Elisee Recluse, La Terre, i., p. 460.]

In fact, the alluvial plains which border the course of most

considerable streams, and are overflowed in their inundations, either by

the rise of the water to a higher level than that of their banks, or by

the bursting of their dikes, serve as safety-valves for the escape of

their superfluous waters. The current of the Po, spreading over the

whole space between its widely separated embankments, takes up so much

water in its inundations, that, while a little below the outlet of the

Ticino the discharge of the channel is sometimes not less than 19,500

cubic yards to the second, it has never exceeded 6,730 yards at Ponte

Lagoscuro, near Ferrara. The currents of the Mississippi, the Rhone, and

of many other large rivers, are modified in the same way. In the flood

of 1858, the delivery of the Mississippi, a little below the month of

the Ohio, was 52,000 cubic yards to the second, but at Baton Rouge,

though of course increased by the waters of the Arkansas, the Yazoo, and

other smaller tributaries, the discharge was reduced to 46,760 cubic

yards. We rarely err when we cautiously imitate the processes of nature,

and there are doubtless many cases where artificial basins of reception

and lateral expansions of river-beds might be employed with advantage.

Many upland streams present points where none of the objections usually

urged against artificial reservoirs, except those of expense and of

danger from the breaking of dams, could have any application. Reservoirs

may be so constructed as to retain the entire precipitation of the

heaviest thaws and rains, leaving only the ordinary quantity to flow

along the channel; they may be raised to such a height as only partially

to obstruct the surface drainage; or they may be provided with sluices

by means of which their whole contents can be discharged in the dry

season and a summer crop be grown upon the ground they cover at high

water. The expediency of employing them and the mode of construction

depend on local conditions, and no rules of universal applicability can

be laid down on the subject. [Footnote: The insufficiency of artificial

basins of reception as a means of averting the evils resulting from the

floods of great rivers has been conclusively shown, in reference to a

most important particular case--that of the Mississippi--by Humphreys

and Abbot, in their admirable monograph of that river.]

It is remarkable that nations which we, in the inflated pride of our

modern civilization, so generally regard as little less than barbarian,

should have long preceded Christian Europe in the systematic employment

of great artificial basins for the various purposes they are calculated

to subserve. The ancient Peruvians built strong walls, of excellent

workmanship, across the channels of the mountain sources of important

streams, and the Arabs executed immense works of similar description,

both in the great Arabian peninsula and in all the provinces of Spain

which had the good fortune to fall under their sway. The Spaniards of

the fifteenth and sixteenth centuries, who, in many points of true

civilization and culture, were far inferior to the races they subdued,

wantonly destroyed these noble monuments of social and political wisdom,

or suffered them to perish, because they were too ignorant to appreciate

their value, or too unskilful as practical engineers to be able to

maintain them, and some of their most important territories were soon

reduced to sterility and poverty in consequence.

Diversion of Rivers.

Another method of preventing or diminishing the evils of inundation by

torrents and mountain rivers, analogous to that employed for the

drainage of lakes, consists in the permanent or occasional diversion of

their surplus waters, or of their entire currents, from their natural

courses, by tunnels or open channels cut through their banks. Nature, in

many cases, resorts to a similar process. Most great rivers divide

themselves into several arms in their lower course, and enter the sea by

different mouths. There are also cases where rivers send off lateral

branches to convey a part of their waters into the channel of other

streams. [Footnote: Some geographical writers apply the term bifurcation

exclusively to this intercommunication of rivers; others, with more

etymological propriety, use it to express the division of great rivers

into branches at the head of their deltas. A technical word is wanting

to designate the phenomenon mentioned in the text, and there is no valid

objection to the employment of the anatomical term anastomosis for this

purpose.] The most remarkable of these is the junction between the

Amazon and the Orinoco by the natural canal of the Cassiquiare and the

Rio Negro. In India, the Cambodja and the Menam are connected by the

Anam; the Saluen and the Irawaddi by the Panlaun. There are similar

examples, though on a much smaller scale, in Europe. The Tornea, and the

Calix rivers in Lapland communicate by the Tarando, and in Westphalia,

the Else, an arm of the Haase, falls into the Weser. [Footnote: The

division of the currents of rivers, as a means of preventing the

overflow of their banks, is by no means a remedy capable of general

application, even when local conditions are favorable to the

construction of an emissary. The velocity of a stream, and consequently

its delivery in a given time, are frequently diminished in proportion to

the diminution of the volume by diversion; and on the other hand, the

increase of volume by the admission of a new tributary increases

proportionally the velocity and the quantity of water delivered.

Emissaries may, nevertheless, often be useful in carrying off water

which has already escaped from the channel and which would otherwise

become stagnant and prevent further lateral discharge from the main

current, and it is upon this principle that Humphreys and Abbot think a

canal of diversion at Lake Providence might be advisable. Emissaries

serve an important purpose in the lower course of rivers where the bed

is nearly a dead level and the water moves from previously acquired

momentum and the pressure of the current above, rather than by the force

of gravitation, and it is, in general, only under such circumstances, as

for example in the deltas at the mouths of great rivers, that nature

employs them.]

The change of bed in rivers by gradual erosion of their banks is

familiar to all, but instances of the sudden abandonment of a primitive

channel are by no means wanting. At a period of unknown antiquity, the

Ardeche pierced a tunnel 200 feet wide and 100 high, through a rock, and

sent its whole current through it, deserting its former bed, which

gradually filled up, though its course remained traceable. In the great

inundation of 1827, the tunnel proved insufficient for the discharge of

the water, and the river burst through the obstructions which had now

choked up its ancient channel, and resumed its original course.

[Footnote: Mardigny, Memoire sur les Inondations de l'Ardeche, p. 13.]

It was probably such facts as these that suggested to ancient engineers

the possibility of like artificial operations, and there are numerous

instances of the execution of works for this purpose in very remote

ages. The Bahr Jusef, the great stream which supplies the Fayoum with

water from the Nile, has been supposed, by some writers, to be a natural

channel; but both it and the Bahr el Wady are almost certainly

artificial canals constructed to water that basin, to regulate the level

of Lake Meeris, and possibly, also, to diminish the dangers resulting

from excessive inundations of the Nile, by serving as waste-weirs to

discharge a part of its overflowing waters. [Footnote: The

starting-points of these anals were far up the Nile, and of course at a

comparatively high level, and it is probable that they received water

only during the inundation. Linant Bey calculates the capacity of Lake

Moeris at 3,686,667 cubic yards and the water received by it at high

Nile at 465 cubic yards the second.] Several of the seven ancient mouths

of the Nile are believed to be artificial channels, and Herodotus even

asserts that King Menes diverted the entire course of that river from

the Libyan to the Arabian side of the valley. There are traces of an

ancient river-bed along the western mountains, which give eome

countenance to this statement. But it is much more probable that the

works of Menes were designed rather to prevent a natural, than to

produce an artificial, change in the channel of the river.

Two of the most celebrated cascades in Europe, those of the Teverone at

Tivoli and of the Velino at Terni, owe, if not their existence, at least

their position and character, to the diversion of their waters from

their natural beds into new channels, in order to obviate the evils

produced by their frequent floods. Remarkable works of the same sort

have been executed in Switzerland, in very recent times. Until the year

1714, the Kander, which drains several large Alpine valleys, ran, for a

considerable distance, parallel with the Lake of Thun, and a few miles

below the city of that name emptied into the river Aar. It frequently

flooded the flats along the lower part of its course, and it was

determined to divert it into the Lake of Thun. For this purpose, two

parallel tunnels were cut through the intervening rock, and the river

turned into them. The violence of the current burst up the roof of the

tunnels, and, in a very short time, wore the new channel down not less

than one hundred feet, and even deepened the former bed at least fifty

feet, for a distance of two or three miles above the tunnel. The lake

was two hundred feet deep at the point where the river was conducted

into it, but the gravel and sand carried down by the Kander has formed

at its mouth a delta containing more than a hundred acres, which is

still advancing at the rate of several yards a year. The Linth, which

formerly sent its waters directly to the Lake of Zurich, and often

produced very destructive inundations, was turned into the Wallensee

about fifty years ago, and in both these cases a great quantity of

valuable land was rescued both from flood and from insalubrity.

Glacier Lakes.

In Switzerland, the most terrible inundations often result from the

damming up of deep valleys by ice-slips or by the gradual advance of

glaciers, and the accumulation of great masses of water above the

obstructions. The ice is finally dissolved by the heat of summer or the

flow of warm waters, and when it bursts, the lake formed above is

discharged almost in an instant, and all below is swept down to certain

destruction. In 1595, about a hundred and fifty lives and a great amount

of property were lost by the eruption of a lake formed by the descent of

a glacier into the valley of the Drance, and a similar calamity laid

waste a considerable extent of soil in the year 1818. On this latter

occasion, the barrier of ice and snow was 3,000 feet long, 600 thick,

and 400 high, and the lake which had formed above it contained not less

than 800,000,000 cubic feet. A tunnel was driven through the ice, and

about 300,000,000 cubic feet of water safely drawn off by it, but the

thawing of the walls of the tunnel rapidly enlarged it, and before the

lake was half drained, the barrier gave way and the remaining

500,000,000 cubic feet of water were discharged in half an hour. The

recurrence of these floods has since been prevented by directing streams

of water, warmed by the sun, upon the ice in the bed of the valley, and

thus thawing it before it accumulates in sufficient mass to form a new

barrier and threaten serious danger. [Footnote: In 1845 a similar lake

was formed by the extension of the Vernagt glacier. When the ice barrier

gave way, 3,000,000 cubic yards of water were discharged in an

hour.--Sonklar, Die Oetzthaler Gebirgsgruppe, section 167.] In the cases

of diversion of streams above mentioned, important geographical changes

have been directly produced by those operations. By the rarer process of

draining glacier lakes, natural eruptions of water, which would have

occasioned not less important changes in the face of the earth, have

been prevented by human agency. River Embankments. The most obvious and

doubtless earliest method of preventing the escape of river-waters from

their natural channels, and the overflow of fields and towns by their

spread, is that of raised embankments along their course. [Footnote:

Riparian embankments are a real, if not a conscious, imitation of a

natural process. The waters of rivers which flow down planes of gentle

inclination deposit, in their inundations, the largest proportion of

their sediment as soon as, by overflowing their banks, they escape from

the swift current of the channel. The immediate borders of such rivers

consequently become higher than the grounds lying further from the

stream, and constitute, of themselves, a sort of natural dike of small

elevation. In the "intervales" or "bottoms" of the great North American

rivers the alluvial banks are elevated and dry, the flats more remote

from the river lower and swampy. This is generally observable in Egypt

(see Figari Bey, Studi Scientifici sull' Egitto, i, p. 87), though less

so than in the valley of the Mississippi, where the alluvial banks form

natural glacis, descending as you recede from the river, and in some

places, as below Cape Girardeau, at the rate of seven feet in the first

mile. Humphreys and Abbott, Report, pp. 96, 97.

In fact, rivers, like mountain torrents, often run for a long distance

on the summit of a ridge built up by their own deposits. The delta of

the Mississippi is a regular cone, or rather mountain, of dejection,

extending far out into the Gulf of Mexico, along the crest of which the

river flows, sending off here and there, as it approaches the sea, a

system of lateral streams resembling the fan-shaped discharge of a

torrent.] The necessity of such embankments usually arises from the

gradual elevation of the bed of running streams in consequence of the

deposit of the earth and gravel they are charged with in high water;

and, as we have seen, this elevation is rapidly accelerated when the

highlands around the headwaters of rivers are cleared of their forests.

When a river is embanked at a given point, and, consequently, the water

of its floods, which would otherwise spread over a wide surface, is

confined within narrow limits, the velocity of the current and its

transporting power are augmented, and its burden of sand and gravel is

deposited at some lower point, where the rapidity of its flow is checked

by a dam or other artificial obstruction, by a diminution in the

inclination of the bed, by a wider channel, or finally by a lacustrine

or marine basin which receives its waters. Wherever it lets fall solid

material, its channel is raised in consequence, and the declivity of the

whole bed between the head of the embankment and the slack of the stream

is reduced. Hence the current, at first accelerated by confinement, is

afterwards checked by the mechanical resistance of the matter deposited,

and by the diminished inclination of its channel, and then begins again

to let fall the earth it holds in suspension, and to raise its bed at

the point where its overflow had been before prevented by embankment.

[Footnote: In proportion as the dikes are improved, and breaches and the

escape of the water through them are less frequent, the height of the

annual inundations is increased. Some towns on the banks of the Po, and

of course within the system of parallel embankments, were formerly

secure from flood by the height of the artificial mounds on which they

were built; but they have recently been obliged to construct ring-dikes

for their protection.

Lombardini lays down the following general statement of the effects of

river embankments:

"The immediate effect of embanking a river is generally an increase in

the height of its floods, but, at the same time, a depression of its

bed, by reason of the increased force, and consequently excavating

action, of the current.

"It is true that coarser material may hence be carried further, and at

the same time deposit itself on a reduced slope.

"The embankment of the upper branches of a river increases the volume,

and therefore the height of the floods in the lower course, in

consequence of the more rapid discharge of its affluents into it.

"When, in consequence of the flow of a river channel through an alluvial

soil not yet REGULATED, or, in other words, which has not acquired its

normal inclination, the course of the river has not become established,

it is natural that its bed should rise more rapidly after its

embankment. ...

"The embankment of the lower course of a river, near its discharge into

the sea, causes the elevation of the bed of the next reach above, both

because the swelling of the current, in consequence of its lateral

confinement, occasions eddies, and of course deposits, and because the

prolongation of the course of the stream, or the advance of its delta

into the sea, is accelerated."--Dei congiamenti cia soggiacque

l'idraulica condizione del Po, etc., pp. 41, 42.

Del Noce states that in the levellings for the proposed Leopolda

railway, he found that the bed of the Sieue had been permanently

elevated two yards between 1708 and 1844, and that of the Fosso di San

Gaudenzio more than a yard and a half between 1752 and 1845. Those,

indeed, are not rivers of the rank of the Po; but neither are they what

are technically called torrents or mountain streams, whose flow is only

an occasional effect of heavy rains or melting snow.--Trattato delle

Macchie e Foreste di Tuscana, Firenze, 1857, p. 29.] The bank must now

be raised in proportion, and these processes would be repeated and

repeated indefinitely, had not nature provided a remedy in floods, which

sweep out recent deposits, burst the bonds of the river and overwhelm

the adjacent country with final desolation, or divert the current into a

new channel, destined to become, in its turn, the scene of a similar

struggle between man and the waters. [Footnote: The Noang-ho has

repeatedly burst its dikes and changed the channel of its lower course,

sometimes delivering its waters into the sea to the north, sometimes to

the south of the peninsula of Chan-tung, thus varying its point of

discharge by a distance of 220 miles.--Elisee Reclus, La Terre, t. i, p.

477.

Sec interesting notices of the lower course of the Noang-ho in Nature,

Nov. 25, 1869.

The frequent changes of channel and mouth in the deltas of great rivers

are by no means always an effect of diking. The mere accumulation of

deposits in the beds of rivers which transport much sediment compels

them continually to seek new outlets, and it is only by great effort

that art can keep their points of discharge pproximately constant. The

common delta of the Ganges and the Brahmapootra is in a state of

incessant change, and the latter river is said to have shifted its main

channel 200 miles to the west since 1785, the revolution having been

principally accomplished between 1810 and 1830.]

But here, as in so many other fields where nature is brought into

conflict with man, she first resists his attempts at interference with

her operations, then, finding him the stronger, quietly submits to his

rule, and ends by contributing her aid to strengthen the walls and

shackles by which he essays to confine her. If, by assiduous repair of

his dikes, he, for a considerable time, restrains the floods of a river

within new bounds, nature, by a series of ingenious compensations,

brings the fluctuating bed of the stream to a substantially constant

level, and when his ramparts have been, by his toil, raised to a certain

height and widened to a certain thickness, she, by her laws of

gravitation and cohesion, consolidates their material until it becomes

almost as hard, as indissoluble, and as impervious as the rock.

But, though man may press the forces of nature into his service, there

is a limit to the extent of his dominion over them, and unless future

generations shall discover new modes of controlling those forces, or new

remedies against their action, he must at last succumb in the struggle.

When the marine estuaries and other basins of reception shall be filled

up with the sedimentary debris of the mountains, or when the lower

course of the rivers shall be raised or prolonged by their own deposits

until they have, no longer, such a descent that gravitation and the

momentum of the current can overcome the frictional resistance of the

bed and banks, the water will, in spite of all obstacles, diffuse itself

laterally and for a time raise the level of the champaign land upon its

borders, and at last convert it into morasses. It is for this reason

that Lombardini advises that a considerable space along the lower course

of rivers be left undiked, and the water allowed to spread itself over

its banks and gradually raise them by its deposits. [Footnote: This

method has been adopted on the lower course of the Lamone, and a

considerable extent of low ground adjacent to that river has been raised

by spontaneous deposit to a sufficient height to admit of profitable

cultivation.] This would, indeed, be a palliative, but only a

palliative. For the present, however, we have nothing better, and here,

as often in political economy, we must content ourselves with "apres

nous le deluge," allowing posterity to suffer the penalty of our

improvidence and our ignorance, or to devise means for itself to ward

off the consequences of them.

The deposit of slime by rivers upon the flats along their banks not only

contributes greatly to the fertility of the soil thus flowed, but it

subserves a still more important purpose in the general economy of

nature. All running streams begin with excavating channels for

themselves, or deepening the natural depressions in which they flow;

[Footnote: I do not mean to say that all rivers excavate their own

valleys, for I have no doubt that in the majority of cases such

depressions of the surface originate in higher geological causes, such

as the fissures and other irregularities of surface which could not fail

to accompany upheaval, and hence the valley makes the river, not the

river the valley. But even if we suppose a basin of the hardest rock to

be elevated at once, completely formed, from the submarine abyss where

it was fashioned, the first shower of rain that falls upon it, after it

rises to the air, will discharge its waters along the lowest lines of

the surface, and cut those lines deeper, and so on with every successive

rain. The disintegrated rock from the upper part of the basin forms the

lower by alluvial deposit, which is constantly transported farther and

farther until the resistance of gravitation and cohesion balances the

mechanical force of the running water. Thus plains, more or less steeply

inclined, are formed, in which the river is constantly changing its bed,

according to the perpetually varying force and direction of its

currents, modified as they are by ever-fluctuating conditions. Thus the

Po is said to have long inclined to move its channel southwards, at

certain points, in consequence of the mechanical force of its northern

affluents. A diversion of these tributaries from their present beds, so

that they should enter the main stream at other points and in different

directions, might modify the whole course of that great river. But the

mechanical force of the tributary is not the only element of its

influence on the course of the principal stream. The deposits it lodges

in the bed of the latter, acting as simple obstructions or causes of

diversion, are not less important agents of change.] but in proportion

as their outlets are raised by the solid material transported by their

currents, their velocity is diminished, they deposit gravel and sand at

constantly higher and higher points, and so at last elevate, in the

middle and lower part of their course, the beds they had previously

scooped out. [Footnote: The distance to which a new obstruction to the

flow of a river, whether by a dam or by a deposit in its channel, will

retard its current, or, in popular phrase, "set back the water," is a

problem of more difficult practical solution than almost any other in

hydraulics. The elements--such as straightness or crookedness of

channel, character of bottom and banks, volume and previous velocity of

current, mass of water far above the obstruction, extraordinary drought

or humidity of seasons, relative extent to which the river may be

affected by the precipitation in its own basin, and by supplies received

through subterranean channels from sources so distant as to be exposed

to very different meteorological influences, effects of clearing and

other improvements always going on in new countries--are all extremely

difficult, and some of them impossible, to be known and measured. In the

American States, very numerous water-mills have been erected within a

few years, and there is scarcely a stream in the settled portion of the

country which has not several mill-dams upon it. When a dam is raised--a

process which the gradual diminution of the summer currents renders

frequently necessary--or when a new dam is built, it often happens that

the meadows above are flowed, or that the retardation of the stream

extends back to the dam next above. This leads to frequent law-suits.

From the great uncertainty of the facts, the testimony is more

conflicting in these than in any other class of cases, and the obstinacy

with which "water causes" are disputed has become proverbial.] The

raising of the channels is compensated in part by the simultaneous

elevation of their banks and the flats adjoining them, from the deposit

of the finer particles of earth and vegetable mould brought down from

the mountains, without which elevation the low grounds bordering all

rivers would be, as in many cases they in fact are, mere morasses.

All arrangements which tend to obstruct this process of raising the

flats adjacent to the channel, whether consisting in dikes which confine

the waters, and, at the same time, augment the velocity of the current,

or in other means of producing the last-mentioned effect, interfere with

the restorative economy of nature, and at last occasion the formation of

marshes where, if left to herself, she might have accumulated

inexhaustible stores of the richest soil, and spread them out in plains

above the reach of ordinary floods. [Footnote: The sediment of the Po

has filled up some lagoons and swamps in its delta, and converted them

into comparatively dry land; but, on the other hand, the retardation of

the current from the lengthening of its course, and the diminution of

its velocity by the deposits at its mouth, have forced its waters at

some higher points to spread in spite of embankments, and thus fertile

fields have been turned into unhealthy and unproductive marshes.--See

Botter, Sulla condizione dei Terreni Maremmani nel Ferraress. Annali di

Agricoltura, etc., Fasc. v., 1863.]

Dikes, which, as we have seen, are the means most frequently employed to

prevent damage by inundation, are generally parallel to each other and

separated by a distance not very much greater than the natural width of

the bed. [Footnote: In the case of rivers flowing through wide alluvial

plains and much inclined to shift their beds, like the Po, the

embankments often leave a very wide space between them. The dikes of the

Po are sometimes three or four miles apart.] If such walls are high

enough to confine the water and strong enough to resist its pressure,

they secure the lands behind them from all the evils of inundation

except those resulting from filtration; but such ramparts are enormously

costly in original construction and in maintenance, and, as has been

already shown, the filling up of the bed of the river in its lower

course, by sand and gravel, often involves the necessity of incurring

new expenditures in increasing the height of the banks. [Footnote: It

appears from the investigations of Lombardini that the rate of elevation

of the bed of the Po has been much exaggerated by earlier writers, and

in some parts of its course the change is so slow that its level may be

regarded as nearly constant. Observation has established a similar

constancy in the bed of the Rhone and of many other important rivers,

while, on the other hand, the beds of the Adige and the Brenta, streams

of a more torrential character, are raised considerably above the level

of the adjacent fields.

The length of the lower course of the Po having been considerably

increased by the filling up of the Adriatic with its deposits, the

velocity of the current ought, prima facie, to have been diminished and

its bed raised in proportion. There are abundant grounds for believing

that this has happened in the case of the Nile, and one reason why the

same effect has not been more sensibly perceptible in the Po is, that

the confinement of the current by continuous embankements gives it a

high-water velocity sufficient to sweep out deposits let fall at lower

stages and slower movements of the water. Torrential streams tend to

excavate or to raise their beds according to the inclination, and to the

character of the material they transport. No general law on this point

can be laid down in relation to the middle and lower courses of rivers.

The conditions which determine the question of the depression or

elevation of a river-bed are too multifarious, variable, and complex, to

be subjected to formulae, and they can scarcely even be enumerated.

The following observation, however, though apparently too

unconditionally stated, is too important to be omitted.

Rivers which transport sand, gravel, pebbles, heavy mineral matter in

short, tend to raise their own beds; those charged only with fine, light

earth, to cut them deeper. The prairie rivers of the western United

States have deep channels, because the mineral matter they carry down is

not heavy enough to resist the impulse of even a moderate current, and

those tributaries of the Po which deposit their sediment in the

lakes--the Ticino, the Adda, the Oglio, and the Mincio--flow in deep

cuts, for the same reason.--Baumgarten, p. 132.

In regard to the level of the bed of the Po, there is another weighty

consideration which does not seem to have received the attention it

deserves. refer to the secular depression of the western coast of the

Adriatic, which is computed at the rate of fifteen or twenty centimetres

in a century, and which of course increases the inclination of the bed,

and the velocity and transporting power of the current of the Po, UNLESS

we assume that the whole course of the river, from the sea to its

sources, shares in the depression. Of this assumption there is no proof,

and the probability is to the contrary. For the evidence, though not

conclusive, perhaps, tends to show an elevation of the Tuscan coast, and

even of the Ligurian shore at points lying farther west than the sources

of the Po. The level of certain parts of the bed of the river referred

to by Lombardini as constant, is not their elevation as compared with

points nearer the sea, but relatively to the adjacent plains, and there

is every reason to believe that the depression of the Adriatic coast,

whether, as is conceivable, occasioned by the mere weight of the

fluviatile deposits or by more general geological causes, has increased

the slope of the bed of the river between the points in question and the

sea. In this instance, then, the relative permanency of the river level

at certain points may be, not the ordinary case of a natural

equilibrium, but the negative effect of an increased velocity of current

which prevents deposits where they would otherwise have happened.] They

are attended, too, with some collateral disadvantages. They deprive the

earth of the fertilizing deposits of the waters, which are powerful

natural restoratives of soils exhausted by cultivation; they accelerate

the rapidity and transporting power of the current at high water by

confining it to a narrower channel, and it consequently conveys to the

sea the earthy matter it holds in suspension, and chokes up harbors with

a deposit which it would otherwise have spread over a wider surface;

they interfere with roads and the convenience of river navigation, and

no amount of cost or care can secure them from occasional rupture, in

case of which the rush of the waters through the breach is more

destructive than the natural flow of the highest inundation. [Footnote:

To secure the city of Sacramento, in California, from the inundations to

which it is subject, a dike or levee was built upon the bank of the

river and raised to an elevation above that of the highest known floods,

and it was connected, below the town, with grounds lying considerably

above the river. On one occasion a breach in the dike occurred above the

town at a very high stage of the flood. The water poured in behind it,

and overflowed the lower part of the city, which remained submerged for

some time after the river had retired to its ordinary level, because the

dike, which had been built to keep the water OUT, now kept it IN.

According to Arthur Young, on the lower Po, where the surface of the

river at high water has been elevated considerably above the level of

the adjacent fields by diking, the peasants in his time frequently

endeavored to secure their grounds against threatened devastation

through the bursting of the dikes, by crossing the river when the danger

became imminent and opening a cut in the opposite bank, thus saving

their own property by flooding their neighbors'. He adds, that at high

water the navigation of the river was absolutely interdicted, except to

mail and passenger boats, and that the guards fired upon all others; the

object of the prohibition being to prevent the peasants from resorting

to this measure of self-defence.--Travels in Italy and Spain, Nov. 7,

1789.

In a flood of the Po in 1839, a breach of the embankment took place at

Bonizzo. The water poured through and inundated 116,000 acres, or 181

square miles, of the plain to the depth of from twenty to twenty-three

feet, in the lower parts. The inundation of May, 1872, a giant breach

occurred in the dike near Ferrara, and 170,000 acres of cultivated land

were overflowed, and a population of 30,000 souls driven from their

homes. In the flood of October in the same year, in consequence of a

breach of the dike at Revere, 250,000 acres of cultivated soil were

overflowed, and 60,000 persons were made homeless. The dikes were

seriously injured at more than forty points. See page 279, ante. In the

flood of 1856, the Loire made seventy-three breaches in its dikes, and

thus, instead of a comparatively gradual rise and gentle expansion of

its waters, it created seventy-three impetuous torrents, which inflicted

infinitely greater mischief than a simply natural overflow would have

done. The dikes or levees of the Mississippi, being of more recent

construction than those of the Po, are not yet well consolidated and

fortified, and for this reason crevasses which occasion destructive

inundations are of very frequent occurrence.]

For these reasons, many experienced engineers are of opinion that the

system of longitudinal dikes is fundamentally wrong, and it has been

argued that if the Po, the Adige, and the Brenta had been left

unconfined, as the Nile formerly was, and allowed to spread their muddy

waters at will, according to the laws of nature, the sediment they have

carried to the coast would have been chiefly distributed over the plains

of Lombardy. Their banks, it is supposed, would have risen as fast as

their beds, the coast-line would not have been extended so far into the

Adriatic, and, the current of the streams being consequently shorter,

the inclination of their channel and the rapidity of their flow would

not have been so greatly diminished. Had man, too, spared a reasonable

proportion of the forests of the Alps, and not attempted to control the

natural drainage of the surface, the Po, it has been said, would

resemble the Nile in all its essential characteristics, and, in spite of

the difference of climate, perhaps be regarded as the friend and ally,

not the enemy and the invader, of the population which dwells upon its

banks.

But it has been shown by Humphreys and Abbot that the system of

longitudinal dikes is the only one susceptible of advantageous

application to the Mississippi, and if we knew the primitive geography

and hydrography of the basin of the Po as well as wo do those of the

valley of the great American river, we should very probably find that

the condemnation of the plan pursued by the ancient inhabitants of

Lombardy is a too hasty generalization, and that the case of the Nile is

an exception, not an example of the normal regime and condition of a

great river. [Footnote: Embankments have been employed on the lower

course of the Po for at least two thousand years, and for some centuries

they have been connected in a continuous chain from the sea to the

vicinity of Cremona. From early ages the Italian hydrographers have

stood in the front rank of their profession, and the Italian literature

of this branch of material improvement is exceedingly voluminous,

exhaustive, and complete.

"The science of rivers after the barbarous ages," says Mengotti, "may be

said to have been born and perfected in Italy." The eminent Italian

engineer Lombardini published in 1870, under the title of Guida allo

studio dell' idrologia fluviale e dell' Idraulica practica, which serves

both as a summary of the recent progress of that science and as an index

to the literature of the subject. The professional student, therefore,

as well as the geographer, will have very frequent occasion to consult

Italian authorities, and in the very valuable Report of Humphreys and

Abbot on the Mississippi, America has lately made a contribution to our

potamological knowledge, which, in scientific interest and practical

utility, does not fall short of the ablest European productions in the

same branch of inquiry.]

But in any event, these theoretical objections are counsels apres coup.

The dikes of the Po and probably of some of its tributaries were begun

before we have any trustworthy physical or political annals of the

provinces they water. The civilization of the valley has accommodated

itself to these arrangements, and the interests which might be

sacrificed by a change of system are too vast to be hazarded by what, in

the present state of our knowledge, can be only considered as a doubtful

experiment. [Footnote: Dupenchel advised a resort to the "heroic remedy"

of sacrificing, or converting into cellars, the lower storeys of houses

in cities exposed to river inundation, filling up the streets, and

admitting the water of floods freely over the adjacent country, and thus

allowing it to raise the level of the soil to that of the highest

inundations.--Traite d'Hydraulique et de Geologie Agricole, Paris, 1868,

p. 241.]

The embankments of the Po, though they are of vast extent and have

employed centuries in their construction, are inferior in magnitude to

the dikes or levees of the Mississippi, which are the work of scarcely a

hundred years, and of a comparatively sparse population. On the right or

western bank of the river, the levee extends, with only occasional

interruptions from high bluffs and the mouths of rivers, for a distance

of more than eleven hundred miles. The left bank is, in general, higher

than the right, and upon that side a continuous embankment is not

needed; but the total length of the dikes of the Mississippi, including

those of the lower course of its tributaries and of its bayous or

natural emissaries, is not less than 2,500 miles. They constitute,

therefore, not only one of the greatest material achievements of the

American people, but one of the most remarkable systems of physical

improvement which has been anywhere accomplished in modern times.

Those who condemn the system of longitudinal embankments have often

advised that, in cases where that system cannot be abandoned without

involving too great a sacrifice of existing interests, the elevation of

the dikes should be much reduced, so as to present no obstruction to the

lateral spread of extraordinary floods, and that they should be provided

with sluices to admit the water without violence whenever they are

likely to be overflowed. Where dikes have not been erected, or where

they have been reduced in height, it is proposed to construct, at

convenient intervals, transverse embankments of moderate height running

from the banks of the river across the plains to the hills which bound

them. These measures, it is argued, will diminish the violence of

inundations by permitting the waters to extend themselves over a greater

surface, and by thus retarding the flow of the river currents, will, at

the same time, secure the deposit of fertilizing slime upon all the soil

covered by the flood. [Footnote: The system described in the text is

substantially the Egyptian method, the ancient Nile dikes having been

constructed rather to retain than to exclude the water.]

Rozet, an eminent French engineer, has proposed a method of diminishing

the ravages of inundations, which aims to combine the advantages of all

other systems, and at the same time to obviate the objections to which

they are all more or less liable. [Footnote: Moyens de forcer les

Torrents de rendre une partie du sol qu'ils ravagent, et d'empecher les

grandes Inondations.] The plan of Rozet is recommended by its simplicity

and cheapness as well as its facility and rapidity of execution, and is

looked upon with favor by many persons very competent to judge in such

matters. It is, however, by no means capable of universal application,

though it would often doubtless prove highly useful in connection with

the measures now employed in South-eastern France. He proposes to

commence with the amphitheatres in which mountain torrents so often

rise, by covering their slopes and filling their beds with loose blocks

of rock, and by constructing at their outlets, and at other narrow

points in the channels of the torrents, permeable barriers of the same

material promiscuously heaped up, much according to the method employed

by the ancient Romans in their northern provinces for a similar purpose.

By this means, he supposes, the rapidity of the current would be

checked, and the quantity of transported pebbles and gravel--which, by

increasing the mechanical force of the water, greatly aggravate the

damage by floods--much diminished. When the stream has reached that part

of its course where it is bordered by soil capable of cultivation, and

worth the expense of protection, he proposes to place along one or both

banks, according to circumstances, a line of cubical blocks of stone or

pillars of masonry three or four feet high and wide, and at the distance

of about eleven yards from each other. The space between the two lines,

or between a line and the opposite high bank, would, of course, be

determined by observation of the width of the swift-water current at

high floods. As an auxiliary measure, small ditches and banks, or low

walls of pebbles, should be constructed from the line of blocks across

the grounds to be protected, nearly at right angles to the current, but

slightly inclining downwards, and at convenient distances from each

other. Rozet thinks the proper interval would be 300 yards, and it is

evident that, if he is right in his main principle, hedges, rows of

trees, or even common fences, would in many cases answer as good a

purpose as banks and trenches or low walls. The blocks or pillars of

stone would, he contends, check the lateral currents so as to compel

them to let fall all their pebbles and gravel in the main channel--where

they would be rolled along until ground down to sand or silt--and the

transverse obstructions would detain the water upon the soil long enough

to secure the deposit of its fertilizing slime. Numerous facts are cited

in support of the author's views, and I imagine there are few residents

of rural districts whose own observation will not furnish testimony

confirmatory of their soundness. [Footnote: The effect of trees and

other detached obstructions in checking the flow of water is

particularly noticed by Palissy in his essay on Waters and Fountains, p.

173, edition of 1844. "There be," says he, "in divers parts of France,

and specially at Nantes, wooden bridges, where, to break the force of

the waters and of the floating ice, which might endamage the piers of

the said bridges, they have driven upright timbers into the bed of the

rivers above the said piers, without the which they should abide but

little. And in like wise, the trees which be planted along the mountains

do much deaden the violence of the waters that flow from them."

Lombardini attaches great importance to the planting of rows of trees

transversely to the current on grounds subject to overflow.--Esame degli

Studi sul Tevere, Section 53, and Appendice, Sections 33, 34.]

Removal of Obstructions.

The removal of obstructions in the beds of rivers dredging the bottom or

blasting rocks, the washing out of deposits and locally increasing the

depth of water by narrowing the channel by moans of spurs or other

constructions projecting from the banks, and, finally, the cutting off

of bends and thus shortening the course of the stream, diminishing the

resistance of its shores and bottom and giving the bed a more rapid

declivity, have all been employed not only to facilitate navigation, but

as auxiliaries to more effectual modes of preventing inundations. But a

bar removed from one point is almost sure to re-form at the same or

another, spurs occasion injurious eddies and unforeseen diversions of

the current, [Footnote: The introduction of a new system of spurs with

parabolic curves has been attended with giant advantage in

France.--Annales du Genie Civil, Mai, 1863.] and the cutting off of

bends, though occasionally effected by nature herself, and sometimes

advantageous in torrential streams whose banks are secured by solid

walls of stone or other artificial constructions, seldom establishes a

permanent channel, and besides, the increased rapidity of the flow

through the new cut often injuriously affects the regime of the river

for a considerable distance below. [Footnote: This practice has

sometimes been resorted to on the Mississippi with advantage to

navigation, but it is quite another question whether that advantage has

not been too dearly purchased by the injury to the banks at lower

points. If we suppose a river to have a navigable course of 1,600 miles

as measured by its natural channel, with a descent of 800 feet, we shall

have a fall of six inches to the mile. If the length of channel be

reduced to 1,200 miles by cutting off bends, the fall is increased to

eight inches per mile. The augmentation of velocity consequent upon this

increase of inclination is not computable without taking into account

other elements, such as depth and volume of water, diminution of direct

resistance, and the like, but in almost any supposable case, it would be

sufficient to produce great effects on the height of floods, the deposit

of sediment in the channel, on the shores, and at the outlet, the

erosion of banks and other points of much geographical importance.

The Po, in those parts of its course where the embankments leave a wide

space between, often cuts off bends in its channel and straightens its

course. These short cuts are called salti, or leaps, and sometimes

abridge the distance between their termini by several miles. In 1777,

the salto of Cottaro shortened a distance of 7,000 metres by 5,000, or,

in other words, reduced the length of the river by five kilometres, or

about three miles, and in 1807 and 1810 the two salti of Mozzanone

effected a still greater reduction.]

Combination of Methods.

Upon the whole, it is obvious that no one of the methods heretofore

practised or proposed for averting the evils resulting from river

inundations is capable of universal application. Each of them is

specially suited to a special case. But the hydrography of almost every

considerable river and its tributaries will be found to embrace most

special cases, most known forms of superficial fluid circulation. For

rivers, in general, begin in the mountains, traverse the plains, and end

in the sea; they are torrents at their sources, swelling streams in

their middle course, placid currents, flowing molli flumine, at their

termination. Hence in the different parts of their course the different

methods of controlling and utilizing them may successively find

application, and there is every reason to believe that by a judicious

application of all, every great river may, in a considerable degree, be

deprived of its powers of evil and rendered subservient to the use, the

convenience, and the dominion of man. [Footnote: On the remedies against

inundation, see the valuable paper of Lombardini, Sulle Inondazioni

avvenute in questi ultimi tempi in Francia. Milano, 1858.

There can be no doubt that in the case of rivers which receive their

supply in a large measure from mountain streams, the methods described

in a former chapter as recently employed in South-eastern France to

arrest the formation and lessen the force of torrents, would prove

equally useful as a preventive remedy against inundations. They would

both retard the delivery of surface-water and diminish the discharge of

sediment into rivers, thus operating at once against the two most

efficient causes of destructive floods. See Chapter III., pp. 316 at

seqq.]

Dikes of the Nile.

"History tells us," says Mengotti, "that the Nile became terrible and

destructive to ancient Egypt, in consequence of being confined within

elevated dikes, from the borders of Nubia to the sea. It being

impossible for these barriers to resist the pressure of its waters at

such a height, its floods burst its ramparts, sometimes on one side,

sometimes on the other, and deluged the plains, which lay far below the

level of its current. . . . In one of its formidable inundations the

Nile overwhelmed and drowned a large part of the population. The

Egyptians then perceived that they were struggling against nature in

vain, and they resolved to remove the dikes, and permit the river to

expand itself laterally and raise by its deposits the surface of the

fields which border its channel." [Footnote: Idraulica Fisica e

Sperimentale. 2d edizione, vol. i., pp. 131, 133.]

The original texts of the passages cited by Mengotti, from Latin

translations of Diodorus Siculus and Plutarch and from Pliny the Elder,

do not by any means confirm this statement, though the most important of

them, that from Diodorus Siculus, is, perhaps, not irreconcilable with

it. Not one of them speaks of the removal of the dikes, and I understand

them all as relating to the mixed system of embankments, reservoirs, and

canals which have been employed in Egypt through the whole period

concerning which we have clear information. I suppose that the

disastrous inundations referred to by the authors in question were

simply extraordinary floods of the same character as those which have

been frequent at later periods of Egyptian history, and I find nothing

in support of the proposition that continuous embankments along the

banks of the Nile ever existed until such were constructed by Mehemet

Ali. [Footnote: The gradual elevation of the bed of the Nile from

sedimentary deposit, from the prolongation of the Delta and consequent

reduction of the inclination of the river-bed, or, as has been supposed

by some, though without probability, from a secular rise of the coast,

rendered necessary some change in the hydraulic arrangements of Egypt.

Mehemet Ali was advised to adopt a system of longitudinal levees, and he

embanked the river from Jebel Silsileh to the sea with dikes six or

seven feet high and twenty feet thick. Similar embankments were made

around the Delta. These dikes are provided with transverse embankments,

with sluices for admitting and canals for distributing the water, and

they serve rather to retain the water and control its flow than to

exclude it. Clot Bey, Apercu sur l'Egypte, ii., 437.]

The object of the dikes of the Po, and, with few exceptions, of those of

other European rivers, has always been to confine the waters of floods

and the solid material transported by them within as narrow a channel as

possible, and entirely to prevent them from flowing over the adjacent

plains. The object of the Egyptian dikes and canals is the reverse,

namely, to diffuse the swelling waters and their sediment over as wide a

surface as possible, to store them up until the soil they cover has them

thoroughly saturated and enriched, and then to conduct them over other

grounds requiring a longer or a second submersion, and, in general, to

suffer none of the precious fluid to escape except by evaporation and

infiltration.

Lake Moeris, whether wholly an artificial excavation, or a natural basin

converted by embankments into a reservoir, was designed chiefly for the

same purpose as the barrage built by Mougel Bey across the two great

arms which enclose the Delta, namely, as a magazine to furnish a

perennial supply of water to the thirsty soil. But these artificial

arrangements alone did not suffice. Canals were dug to receive the water

at lower stages of the river and conduct it far into the interior, and

as all this was still not enough, hundreds of thousands of wells were

sunk to bring up from the subsoil, and spread over the surface, the

water which, by means of infiltration from the river-bed, pervades the

inferior strata of the whole valley. [Footnote: It is said that in the

Delta alone 50,000 wells are employed for irrigation.]

If a system of lofty continuous dikes, like those of the Po, had really

been adopted in Egypt, in the early dynasties when the power and the

will to undertake the most stupendous material enterprises were so

eminently characteristic of the government of that country, and

persevered in through later ages, and the waters of the annual

inundation had thus been permanently prevented from flooding the land,

it is conceivable that the productiveness of the small area of

cultivable soil in the Nile valley might have been long kept up by

artificial irrigation and the application of manures. But nature would

have rebelled at last, and centuries before our time the mighty river

would have burst the fetters by which impotent man had vainly striven to

bind his swelling floods, the fertile fields of Egypt would have been

converted into dank morasses, and then, perhaps, in some distant future,

when the expulsion of man should have allowed the gradual restoration of

the primitive equilibrium, would be again transformed into luxuriant

garden and plough land. Fortunately, the sapientia AEgyptiorum, the

wisdom of the Egyptians, taught them better things. They invited and

welcomed, not repulsed, the slimy embraces of Nilus, and his favors have

been, from the hoariest antiquity, the greatest material blessing that

nature ever bestowed upon a people. [Footnote: Deep borings have not

detected any essential difference in the quantity or quality of the

deposits of the Nile for forty or fifty, or, as some compute, for a

hundred centuries. From what vast store of rich earth does this river

derive the three or four inches of fertilizing material which it spreads

over the soil of Egypt every hundred years Not from the White Nile, for

that river drops nearly all its suspended matter in the broad expansions

and slow current of its channel south of the tenth degree of north

latitude. Nor does it appear that much sediment is contributed by the

Bahr-el-Azrek, which flows through forests for a great part of its

course. I have been informed by an old European resident of Egypt who is

very familiar with the Upper Nile, that almost the whole of the earth

with which its waters are charged is brought down by the Takazze.]

Deposits of the Nile.

The Nile is larger than all the rivers of Lombardy together, [Footnote:

From daily measurements during a period of fourteen years--1827 to

1840--the mean delivery of the Po at Ponte Lagoscuro, below the entrance

of its last tributary, is found to be 1,720 cubic metres, or 60,745

cubic feet, per second. Its smallest delivery is 186 cubic metres, or

6,569 cubic feet, its greatest 5,156 cubic metres, or 152,094 cubic

feet. The average delivery of the Nile being 101,000 cubic feet per

second, it follows that the Po contributes to the Adriatic rather more

than six-tenths as much water as the Nile to the Mediterranean--a result

which will surprise most readers.

It is worth remembering that the mean delivery of the Rhone is almost

identical with that of the Po, and that of the Rhine is very nearly the

same. Though the Po receives four-tenths of its water from lakes, in

which the streams that empty into them let fall the solid material they

bring down from the mountains, its deposits in the Adriatic are at least

sixty or seventy per cent. greater than those transported to the

Mediterranean by the Rhone, which derives most of its supply from

mountain and torrential tributaries. Those tributaries lodge much

sediment in the Lake of Geneva and the Lac de Bourget, but the total

erosion of the Po and its affluents must be considerably greater than

that of the Rhone system. The Rhine conveys to the sea much less

sediment than either of the other two rivers.--Lombardini, Cargiamenti

nella condizione del Po, pp. 29, 39.

The mean discharge of the Mississippi is 675,000 cubic feet per second,

and, accordingly, that river contributes to the sea about eleven times

as much water as the Po, and more than six and a half times as much as

the Nile. The discharge of the Mississippi is estimated at one-fourth of

the precipitation in its basin--certainly a very large proportion, when

we consider the rapidity of evaporation in many parts of the basin, and

the probable loss by infiltration.--Humphreys and Abbott'S Report, p.

93.

The basin of the Mississippi has an area forty-six times as large as

that of the Po, with a mean annual precipitation of thirty inches, while

that of the Po, at least according to official statistics, has a

precipitation of forty inches. Hence the down-fall in the former is

one-fourth less than in the latter. Besides this, the Mississippi loses

little or nothing by the diversion of its waters for irrigation.

Consequently the measured discharge of the Mississippi is proportionally

much less than that of the Po, and we are authorized to conclude that

the difference is partly due to the escape of water from the bed, or at

least the basin of the Mississippi, by subterranean channels.

These comparisons are interesting in reference to the supply received by

the sea directly from great rivers, but they fail to give a true idea of

the real volume of the latter. To take the case of the Nile and the Po:

we have reason to suppose that comparatively little water is diverted

from the tributaries of the former for irrigation, but enormous

quantities are drawn from its main trunk for that purpose, below the

point where it receives its last affluent. This quantity is now

increasing in so rapid a proportion, that Elisee Reclus foresees the day

when the entire low-water current will be absorbed by new arrangements

to meet the needs of extended and improved agriculture. On the other

hand, while the affluents of the Po send off a great quantity of water

into canals of irrigation, the main trunk loses little or nothing in

that way except at Chivasso. Trustworthy data are wanting to enable us

to estimate how far these different modes of utilizing the water balance

each other in the case under consideration. Perhaps the Canal Cavour,

and other irrigating canals now proposed, may one day intercept as large

a proportion of the supply of the lower Po as Egyptian dikes, canals,

shadoofs, and steam-pumps do of that of the Nile.

Another circumstance is important to be considered in comparing the

character of these three rivers. The Po runs nearly east and west, and

it and its tributaries are exposed to no other difference of

meterological conditions than those which always subsist between the

mountains and the plains. The course of the Nile and the Mississippi is

mainly north and south. The sources of the Nile are in a very humid

region, its lower course for many hundred miles in almost rainless

latitudes with enormous evaporating power, while the precipitation is

large throughout the Mississippi system, except in the basins of some of

its western affluents.] it drains a basin fifty, possibly even a

hundred, times as extensive, its banks have been occupied by man

probably twice as long. But its geographical character has not been much

changed in the whole period of recorded history, and, though its outlets

have somewhat fluctuated in number and position, its historically known

encroachments upon the sea are trifling compared with those of the Po

and the neighboring streams. The deposits of the Nile are naturally

greater in Upper than in Lower Egypt. They are found to have raised the

soil at Thebes about seven feet within the last seventeen hundred years,

and in the Delta the rise has been certainly more than half as great.

We shall, therefore, probably not exceed the truth if we suppose the

annually inundated surface of Egypt to have been elevated, upon an

average, ten feet, [Footnote: Fraas and Eyth maintain that we have no

trustworthy data for calculating the annual or secular elevation of the

soil of Egypt by the sediment of the Nile. The deposit, they say, is

variable from irregularity of current, and especially from the

interference of man with the operations of nature, to a degree which

renders any probable computation of the amount quite impossible.--Fraas,

Aus dem Orient, pp. 212, 213.

The sedimentary matter transported by the Nile might doubtless be

estimated with approximate precision by careful observation of the

proportion of suspended slime and water at different stations and

seasons for a few successive years. Figari Bey states that at low stages

the water of the Nile contains little or no sediment, and that the

greatest proportion occurs about the end of July, and of course, while

the river is still rising. Experiments at Khartum at that season showed

solid matter in the proportion of one to a thousand by weight. The

quantity is relatively greater at Cairo, a fact which shows that the

river receives more earth from the erosion of its banks than it deposits

at its own bottom, and it must consequently widen its channel unless we

suppose a secular depression of the coast at the mouth of the Nile which

produces an increased inclination of the bed of the river, and

consequently an augmented velocity of flow sufficient to sweep out earth

from the bottom and mix it with the current.

Herschell states the Nile sediment at 1 in 633 by weight, and computes

the entire annual quantity at 140 millions of tons.--Physical Geography,

p. 231.

The mean proportion of sedimentary material in the waters of the

Mississippi is calculated at 1 to 1,500 by weight, and 1 to 2,900 in

volume, and the total annual quantity at 812,500,000,000 pounds, which

would cover one square mile to the depth of 214 feet.--Humphreys and

Abbott, Report, p. 140.] within the last 5,000 years, or twice and a

half the period during which the history of the Po is known to us.

[Footnote: We are quite safe in supposing that the valley of the Nile

has been occupied by man at least 5,000 years. The dates of Egyptian

chronology are uncertain, but I believe no inquirer estimates the age of

the great pyramids at less than forty centuries, and the construction of

such works implies an already ancient civilization.

It is an interesting fact that the old Egyptian system of embankments

and canals is probably more ancient than the geological changes which

have converted the Mississippi from a limpid to a turbid stream, and

occasioned the formation of the vast delta at the mouth of that river.

Humphreys and Abbot conclude that the delta of the Mississippi began its

encroachments on the Gulf of Mexico not more than 4,400 years ago,

before which period they suppose the Mississippi to have been "a

comparatively clear stream," conveying very little sediment to the sea.

The present rate of advance of the delta is 262 feet a year, and there

are reasons for thinking that the amount of deposit has long been

approximately constant.--Report, pp. 435, 436.]

As I have observed, the area of cultivated soil is much less extensive

now than under the dynasties of the Pharaohs and the Ptolemies;

for--though, in consequence of the elevation of the river-bed, the

inundations now have a wider NATURAL spread--the industry of the ancient

Egyptians conducted the Nile water over a great surface which it does

not now reach.

Had the Nile been banked in, like the Po, all this deposit, except that

contained in the water diverted by canals or otherwise drawn from the

river for irrigation and other purposes, would have soon carried out to

sea. This would have been a considerable quantity; for the Nile holds

some earth in suspension at all seasons except at the very lowest water,

a much larger proportion during the flood, and irrigation must have been

carried on during the whole year. The precise amount of sediment which

would have been thus distributed over the soil is matter of conjecture,

but though large, it would have been much less than the inundations have

deposited, and continuous longitudinal embankments would have compelled

the Nile to transport to the Mediterranean an immense quantity over and

above what it has actually deposited in that sea. The Mediterranean is

shoal for some miles out to sea along the whole coast of the Delta, and

the large bays or lagoons within the coast-line, which communicate both

with the river and the sea, have little depth of water. These lagoons

the river deposits would have filled up, and there would still have been

surplus earth enough to extend the Delta far into the Mediterranean.

[Footnote: The present annual extension of the Delta is, if perceptible,

at all events very small. According to some authorities, a few hectares

are added every year at each Nile mouth. Others, among whom I may

mention Fraas, deny that there is any extension at all, the deposit

being balanced by a secular depression of the coast.

Elisee Reclus states that the Delta advances about 40 inches per

year.--La Terre, i., p. 500.]

Obstruction of River Mouths.

The mouths of a large proportion of the streams known to ancient

navigation are already blocked up by sand-bars or fluviatile deposits,

and the maritime approaches to river harbors frequented by the ships of

Phenicia and Carthage and Greece and Rome are shoaled to a considerable

distance out to sea. The inclination of the lower course of almost every

known river bed has been considerably reduced within the historical

period, and nothing but great volume of water, or exceptional rapidity

of flow, now enables a few large streams like the Amazon, the La Plata,

the Ganges, and, in a loss degree, the Mississippi, to carry their own

deposits far enough out into deep water to prevent the formation of

serious obstructions to navigation. But the degradation of their banks,

and the transportation of earthy matter to the sea by their currents,

are gradually filling up the estuaries even of those mighty floods, and

unless the threatened evil shall be averted by the action of geological

forces, or by artificial contrivances more efficient than

dredging-machines, the destruction of every harbor in the world which

receives a considerable river must inevitably take place at no very

distant date.

This result would, perhaps, have followed in some incalculably distant

future, if man had not come to inhabit the earth as soon as the natural

forces which had formed its surface had arrived at such an approximate

equilibrium that his existence on the globe was possible; but the

general effect of his industrial operations has been to accelerate it

immensely. Rivers, in countries planted by nature with forests and never

inhabited by man, employ the little earth and gravel they transport

chiefly to raise their own beds and to form plains in their basins. In

their upper course, where the current is swiftest, they are most heavily

charged with coarse rolled or suspended matter, and this, in floods,

they deposit on their shores in the mountain valleys where they rise; in

their middle course, a lighter earth is spread over the bottom of their

widening basins, and forms plains of moderate extent; the fine silt

which floats farther is deposited over a still broader area, or, if

carried out to sea, is in great part quickly swept far off by marine

currents and dropped at last in deep water. Man's "improvement" of the

soil increased the erosion from its surface; his arrangements for

confining the lateral spread of the water in floods compel the rivers to

transport to their mouths the earth derived from that erosion even in

their upper course; and, consequently, the sediment they deposit at

their outlets is not only much larger in quantity, but composed of

heavier materials, which sink more readily to the bottom of the sea and

are less easily removed by marine currents.

The tidal movement of the ocean, deep-sea currents, and the agitation of

inland waters by the wind, lift up the sands strewn over the bottom by

diluvial streams or sent down by mountain torrents, and throw them up on

dry land, or deposit them in sheltered bays and nooks of the coast--for

the flowing is stronger than the ebbing tide, the affluent than the

refluent wave. This cause of injury to harbors it is not in man's power

to resist by any means at present available; but, as we have seen,

something can be done to prevent the degradation of high grounds, and to

diminish the quantity of earth which is annually abstracted from the

mountains, from table-lands, and from river-banks, to raise the bottom

of the sea.

This latter cause of harbor obstruction, though an active agent, is,

nevertheless, in many cases, the less powerful of the two. The earth

suspended in the lower course of fluviatile currents is lighter than

sea-sand, river water lighter than sea water, and hence, if a land

stream enters the sea with a considerable volume, its water flows over

that of the sea, and bears its slime with it until it lets it fall far

from shore, or, as is more frequently the case, mingles with some marine

current and transports its sediment to a remote point of deposit. The

earth borne out of the mouths of the Nile is in part carried over the

waves which throw up sea-sand on the beach, and deposited in deep

water, in part drifted by the current, which sweeps east and north along

the coasts of Egypt and Syria, and lodged in every nook along the

shore--and among others, to the great detriment of the Suez Canal, in

the artificial harbor at its northern terminus--and in part borne along

until it finds a final resting-place in the north-eastern angle of the

Mediterranean. [Footnote: "The stream carries this mud, etc., at first

farther to the east, and only lets it fall where the force of the

current becomes weakened. This explains the continual advance of the

land seaward along the Syrian coast, in consequence of which Tyre and

Sideon no longer lie on the shore, but some distance inland. That the

Nile contributes to this deposit may easily be seen, even by the

unscientific observer, from the stained and turbid character of the

water for many miles from its mouths. Ships often encounter floating

masses of Nile mud, and Dr. Clarke thus describes a case of this sort:

"While we were at table, we heard the sailors who were throwing the lead

suddenly cry out: 'Three and a half!' The ship slackened her way, and

veered about. As she came round, the whole surface of the water was seen

to be covered with thick, black mud, which extended so far that it

appeared like an island. At the same time, actual land was nowhere to be

seen--not even from the mast-head--nor was any notice of such a shoal to

be found or any chart on board. The fact is, as we learned afterwards,

that a stratum of mud, stretching from the mouths of the Nile for many

miles out into the open sea, forms a movable deposit along the Egyptian

coast. If this deposit is driven forwards by powerful currents, it

sometimes rises to the surface, and disturbs the mariner by the sudden

appearance of shoals where the charts lead him to expect a considerable

depth of water. But these strata of mud are, in reality, not in the

least dangerous. As soon as a ship strikes them they break up at once,

and a frigate may hold her course in perfect safety where an

inexperienced pilot, misled by his soundings, would every moment expect

to be stranded."--Bottger, Das Mittchneer, pp. 188, 189.

This phenomenon is not peculiar to the locality in question, and it is

frequently observed in the Gulf of Bengal, and other great marine

estuaries.]

Thus the earth loosened by the rude Abyssinian ploughshare, and washed

down by the rain from the hills of Ethiopia which man has stripped of

their protecting forests, contributes to raise the plains of Egypt, to

shoal the maritime channels which lead to the city built by Alexander

near the mouth of the Nile, to obstruct the artificial communication

between the Mediterranean and the Red Sea, and to fill up the harbors

made famous by Phenician commerce.

Deposits of the Tuscan Rivers.

The Arno, and all the rivers rising on the western slopes and spurs of

the Apennines, carry down immense quantities of mud to the

Mediterranean. There can be no doubt that the volume of earth so

transported is very much greater than it would have been had the soil

about the headwaters of those rivers continued to be protected from wash

by forests; and there is as little question that the quantity borne out

to sea by the rivers of Western Italy is much increased by artificial

embankments, because they are thereby prevented from spreading over the

surface the sedimentary matter with which they are charged. The western

coast of Tuscany has advanced some miles seawards within a very few

centuries. The bed of the sea, for a long distance, has been raised, and

of course the relative elevation of the land above it lessened; harbors

have been filled up and destroyed; long lines of coast dunes have been

formed, and the diminished inclination of the beds of the rivers near

their outlets has caused their waters to overflow their banks and

convert them into pestilential marshes. The territorial extent of

Western Italy has thus been considerably increased, but the amount of

soil habitable and cultivable by man has been, in a still higher

proportion, diminished. The coast of ancient Etruria was filled with

great commercial towns, and their rural environs were occupied by a

large and prosperous population. But maritime Tuscany has long been one

of the most unhealthy districts in Christendom; the famous Etruscan mart

of Populonia has scarcely an inhabitant; the coast is almost absolutely

depopulated, and the malarious fevers have extended their ravages far

into the interior.

These results are certainly not to be ascribed wholly to human action.

They are, in a large proportion, due to geological causes over which man

has no control. The soil of much of Tuscany becomes pasty, almost fluid

even, as soon as it is moistened, and when thoroughly saturated with

water, it flows like a river. Such a soil as this would not be

completely protected by woods, and, indeed, it would now be difficult to

confine it long enough to allow it to cover itself with forest

vegetation. Nevertheless, it certainly was once chiefly wooded, and the

rivers which flow through it must then have been much less charged with

earthy matter than at present, and they must have carried into the sea a

smaller proportion of their sediment when they were free to deposit it

on their banks than since they have been confined by dikes.

It is, in general, true, that the intervention of man has hitherto

seemed to insure the final exhaustion, ruin, and desolation of every

province of nature which he has reduced to his dominion. Attila was only

giving an energetic and picturesque expression to the tendencies of

human action, as personified in himself, when he said that "no grass

grew where his horse's hoofs had trod." The instances are few, where a

second civilization has flourished upon the ruins of an ancient culture,

and lands once rendered uninhabitable by human acts or neglect have

generally been forever abandoned as hopelessly irreclaimable. It is, as

I have before remarked, a question of vast importance, how far it is

practicable to restore the garden we have wasted, and it is a problem on

which experience throws little light, because few deliberate attempts

have yet been made at the work of physical regeneration, on a scale

large enough to warrant general conclusions in any one class of cases.

The valleys and shores of Tuscany form, however, a striking exception to

this remark. The succcess with which human guidance has made the

operations of nature herself available for the restoration of her

disturbed harmonies, in the Val di Chiana and the Tuscan Maremma, is

among the noblest, if not the most brilliant achievements of modern

engineering, and, regarded in all its bearings on the great question of

which I have just spoken, it is, as an example, of more importance to

the general interests of humanity than the proudest work of internal

improvement that mechanical means have yet constructed. The operations

in the Val di Chiana have consisted chiefly in so regulating the flow of

the surface-waters into and through it, as to compel them to deposit

their sedimentary matter at the will of the engineers, and thereby to

raise grounds rendered insalubrious and unfit for agricultural use by

stagnating water; the improvements in the Maremma have embraced both

this method of elevating the level of the soil, and the prevention of

the mixture of salt-water with fresh in the coast marshes and shallow

bays, which is regarded as a very active cause of the development of

malarious influences. [Footnote: The fact that the mixing of salt and

fresh water in coast marshes and lagoons is deleterious to the sanitary

condition of the vicinity, has been generally admitted, though the

precise reason why a mixture of both should be more injurious than

either alone, is not altogether clear. It has been suggested that the

admission of salt-water to the lagoons and rivers kills many fresh-water

plants and animals, while the fresh water is equally fatal to many

marine organisms, and that the decomposition of the remains originates

poisonous minsmata. Other theories, however, have been proposed. The

whole subject is fully and ably discussed by Dr. Salvagnoli Marchetti in

the appendix to his valuable Rapporto aul Bonificamento delle Maremme

Toscane. See also the Memorie Economico-Statistiche sulle Maremme

Toscane, of the same author. A different view of this subject is taken

by Raffanini and Orlandini in Analisi, Storico-Fisico-Economica sulli

insolubrita nelle Maremme Toscane, Firenze, 1869. See also the important

memoir of D. Pantaleoni, Del miasma vegetale e delle Malattie

Miasmatiche, in which the views of Salvagnoli on this point are

combated.]

Improvements in the Tuscan Maremma.

In the improvements of the Tuscan Maremma, formidable difficulties have

been encountered. The territory to be reclaimed was extensive; the

salubrious places of retreat for laborers and inspectors were remote;

the courses of the rivers to be controlled were long and their natural

inclination not rapid; some of them, rising in wooded regions,

transported comparatively little earthy matter, [Footnote: This

difficulty has been remedied--though with doubtful general advantage--as

to one important river of the Maremma, the Pecora, by clearings recently

executed along its upper course. "The condition of this marsh and of its

affluents are now, November, 1859, much changed, and it is advisable to

prosecute its improvement by deposits. In consequence of the extensive

felling of the woods upon the plains, hills, and mountains of the

territory of Massa and Scarlino, within the last ten years, the Pecora

and other affluents of the marsh receive, during the rains, water

abundantly charged with slime, so that the deposits within the first

division of the marsh are already considerable, and we may now hope to

see the whole marsh and pond filled up in a much shorter time than we

had a right to expect before 1850. This circumstance totally changes the

terms of the question, because the filling of the marsh and pond, which

then seemed almost impossible on account of the small amount of sediment

deposited by the Pecora, has now become practicable."--Salvagnoli,

Rapporto sul Bonificamento delle Maremme Toscane, pp.li., lii.

Between 1830 and 1859 more than 36,000,000 cubic yards of sediment were

deposited in the marsh and shoal-water lake of Castiglione

alone.--Salvagnoli, Raccolta di Documenti, pp. 74, 75.] and above all,

the coast, which is a recent deposit of the waters, is little elevated

above the sea, and admits into its lagoons and the mouths of its rivers

floods of salt-water with every western wind, every rising tide.

[Footnote: The tide rises ten inches on the coast of Tuscany. See Memoir

by Fantoni, in the appendix to Salvagnoli, Rapporto, p. 189.

On the tides of the Mediterranean, see Bottger, Das Mittelmeer, p. 190.]

The western coast of Tuscany is not supposed to have been an unhealthy

region before the conquest of Etruria by the Romans, but it certainly

became so within a few centuries after that event. This was a natural

consequence of the neglect or wanton destruction of the public

improvements, and especially the hydraulic works in which the Etruscans

were so skilful, and of the felling of the upland forests, to satisfy

the demand for wood at Rome for domestic, industrial, and military

purposes. After the downfall of the Roman empire, the incursions of the

barbarians, and then feudalism, foreign domination, intestine wars, and

temporal and spiritual tyrannies, aggravated still more cruelly the

moral and physical evils which Tuscany and the other Italian States were

doomed to suffer, and from which they have enjoyed but brief respites

during the whole period of modern history. The Maremma was already

proverbially unhealthy in the time of Dante, who refers to the fact in

several familiar passages, and the petty tyrants upon its borders often

sent criminals to places of confinement in its territory, as a slow but

certain mode of execution. Ignorance of the causes of the insalubrity,

and often the interference of private rights, [Footnote: In Catholic

countries, the discipline of the church requires a meagre diet at

certain seasons, and as fish is not flesh, there is a great demand for

that article of food at those periods. For the convenience of

monasteries and their patrons, and as a source of pecuniary emolument to

ecclesiastical establishments and sometimes to lay proprietors, great

numbers of artificial fish-ponds were created during the Middle Ages.

They were generally shallow pools formed by damming up the outlet of

marshes, and they were among the most fruitful sources of endemic

disease, and of the peculiar malignity of the epidemics which so often

ravaged Europe in those centuries. These ponds, in religious hands, were

too sacred to be infringed upon for sanitary purposes, and when

belonging to powerful lay lords they were almost an inviolable. The

rights of fishery were a standing obstacle to every proposal of hydralic

improvement, and to this day large and fertile districts in Southern

Europe remain sickly and almost unimproved and uninhabited, because the

draining of the ponds upon them would reduce the income of proprietors

who derive large profits by supplying the faithful, in Lent, with fish,

and with various species of waterfowl which, though very fat, are,

ecclesiastically speaking, meagre.]prevented the adoption of measures to

remove it, and the growing political and commercial importance of the

large towns in more healthful localities absorbed the attention of

Government, and deprived the Maremma of its just share in the systems of

physical improvement which were successfully adopted in interior and

Northern Italy.

Before any serious attempts were made to drain or fill up the marshes of

the Maremma, various other sanitary experiments were tried. It was

generally believed that the insalubrity of the province was the

consequence, not the cause, of its depopulation, and that, if it were

once densely inhabited, the ordinary operations of agriculture, and

especially the maintenance of numerous domestic fires, would restore it

to its ancient healthfulness. [Footnote: Macchiavelli advised the

Government of Tuscany "to provide that men should restore the

wholesomeness of the soil by cultivation, and purify the air by

fires."--Salvagnoli, Memorie, p. 111.] In accordance with these views,

settlers were invited from various parts of Italy, from Greece, and,

after the accession of the Lorraine princes, from that country also, and

colonized in the Maremma. To strangers coming from soils and skies so

unlike those of the Tuscan marshes, the climate was more fatal than to

the inhabitants of the neighboring districts, whose constitutions had

become in some degree inured to the local influences, or who at least

knew better how to guard against them. The consequence very naturally

was that the experiment totally failed to produce the desired effects,

and was attended with a great sacrifice of life and a heavy loss to the

treasury of the state.

The territory known as the Tuscan Maremma, ora maritime, or Maremme--for

the plural form is most generally used--lies upon and near the western

coast of Tuscany, and comprises about 1,900 square miles English, of

which 500 square miles, or 320,000 acres, are plain and marsh including

45,500 acres of water surface, and about 290,000 acres are forest. One

of the mountain peaks, that of Mount Amiata, rises to the height of

6,280 feet. The mountains of the Maremma are healthy, the lower hills

much less so, as the malaria is felt at some points at the height of

1,000 feet, and the plains, with the exception of a few localities

favorably situated on the seacoast, are in a high degree pestilential.

The fixed population is about 80,000, of whom one-sixth live on the

plains in the winter and about one-tenth in the summer. Nine or ten

thousand laborers come down from the mountains of the Maremma and the

neighboring provinces into the plain, during the latter season, to

cultivate and gather the crops.

Out of this small number of inhabitants and strangers, 35,619 were ill

enough to require medical treatment between the 1st of June, 1840, and

the 1st of June, 1841, and more than one-half the cases were of

intermittent, malignant, gastric, or catarrhal fever. Very few

agricultural laborers escaped fever, though the disease did not always

manifest itself until they had returned to the mountains. In the

province of Grosseto, which embraces nearly the whole of the Maremma,

the annual mortality was 3.92 per cent., the average duration of life

but 23.18 years, and 75 per cent. of the deaths were among persons

engaged in agriculture.

The filling up of the low grounds and the partial separation of the

waters of the sea and the land, which had been in progress since the

year 1827, now began to show very decided effects upon the sanitary

condition of the population. In the year ending June 1st, 1842, the

number of the sick was reduced by more than 2,000, and the cases of

fever by more than 4,000. The next year the cases of fever fell to

10,500, and in that ending June 1st, 1844, to 9,200. The political

events of 1848, and the preceding and following years, occasioned the

suspension of the works of improvement in the Maremma, but they were

resumed after the revolution of 1859. I have spoken with some detail of

the improvements in the Tuscan Maremma, because of their great relative

importance, and because their history is well known; but like operations

have been executed in the territory of Pisa and upon the coast of the

duchy of Lucca. In the latter case they were confined principally to

prevention of the intermixing of fresh water with that of the sea. In

1741 sluices or lock-gates were constructed for this purpose, and the

following year the fevers, which had been destructive to the coast

population for a long time previous, disappeared altogether. In 1768 and

1769, the works having fallen to decay, the fevers returned in a very

malignant form, but the rebuilding of the gates again restored the

healthfulness of the shore. Similar facts recurred in 1784 and 1785, and

again from 1804 to 1821. This long and repeated experience has at last

impressed upon the people the necessity of vigilant attention to the

sluices, which are now kept in constant repair. The health of the coast

is uninterrupted, and Viareggio, the capital town of the district, is

now much frequented for its sea-baths and its general salubrity, at a

season when formerly it was justly shunned as the abode of disease and

death. [Footnote: Giorgini, Sur les causes de l'Insalubrite de l'air

dans le voisinage des marais, etc., lue a l'Academie des Sciences a

Paris, le 12 Juillet, 1825. Reprinted in Salvagnoli, Rapporto, etc.,

appendice, p. 5, et seqq.]

Improvements in the Val di Chiana.

For twenty miles or more after the remotest headwaters of the Arno have

united to form a considerable stream, this river flows south-eastwards

to the vicinity of Arezzo. It here sweeps round to the north-west, and

follows that course to near its junction with the Sieve, a few miles

above Florence, from which point its general direction is westward to

the sea. From the bend at Arezzo, a depression called the Val di Chiana

runs south-eastwards until it strikes into the valley of the Paglia, a

tributary of the Tiber, and thus connects the basin of the latter river

with that of the Arno. In the Middle Ages, and down to the eighteenth

century, the Val di Chiana was often overflowed and devastated by the

torrents which poured down from the highlands, transporting great

quantities of slime with their currents, stagnating upon its surface,

and gradually converting it into a marshy and unhealthy district, which

was at last very greatly reduced in population and productiveness. It

had, in fact, become so desolate that even the swallow had deserted

it. [Footnote: This curious fact is thus stated in the preface to

Fossombroni (Memorie sopra la Val di Chiana, edition of 1835, p. xiii.),

from which also I borrow most of the data hereafter given with respect

to that valley: "It is perhaps not universally known, that the swallows,

which come from the north [south] to spend the summer in our climate, do

not frequent marshy districts with a malarious atmosphere. A proof of

the restoration of salubrity in the Val di Chiana is furnished by these

aerial visitors, which had never before been seen in those low grounds,

but which have appeared within a few years at Forano and other points

similarly situated."

Is the air of swamps destructive to the swallows, or is their absence in

such localities merely due to the want of human habitations, near which

this half-domestic bird loves to breed, perhaps because the house-fly

and other insects which follow man are found only in the vicinity of his

dwellings In almoust all European countries the swallow is protected, by

popular opinion or superstition, from the persecution to which almost

all other birds are subject. It is possible that this respect for the

swallow is founded upon ancient observation of the fact just stated on

the authority of Fossombroni. Ignorance mistakes the effect for the

cause, and the absence of this bird may have been supposed to be the

occasion, not the consequence, of the unhealthiness of particular

localities. This opinion once adopted, the swallow would become a sacred

bird, and in process of time fables and legends would be invented to

give additional sanction to the prejudices which protected it. The

Romans considered the swallow as consecrated to the Penates, or

household gods, and according to Peretti (Le Serate del Villaggio, p.

168) the Lombard peasantry think it a sin to kill them, because they are

le gallinelle del Signore, the chickens of the Lord.]

The bed of the Arno near Arezzo and that of the Paglia at the southern

extremity of the Val di Chiana did not differ much in level. The general

inclination of the valley was therefore small; it does not appear to

have ever been divided into opposite slopes by a true watershed, and the

position of the summit seems to have shifted according to the varying

amount and place of deposit of the sediment brought down by the lateral

streams which emptied into it. The length of its principal channel of

drainage, and even the direction of its flow at any given point, were

therefore fluctuating. Hence, much difference of opinion was entertained

at different times with regard to the normal course of this stream, and,

consequently, to the question whether it was to be regarded as properly

an affluent of the Tiber or of the Arno.

The bed of the latter river at the bend has been eroded to the depth of

thirty or forty feet, and that, apparently, at no very remote

period. [Footnote: Able geologists infer from recent investigations,

that, although the Arno flowed to the south within the pliocenic period,

the direction of its course was changed at an earlier epoch than that

supposed in the text.] If it were elevated to what was evidently

original height, the current of the Arno would be so much above that of

the Paglia as to allow of a regular flow from its channel to the latter

stream, through the Val di Chiana, provided the bed of the valley had

remained at the level which excavations prove it to have had a few

centuries ago, before it was raised by the deposits I have mentioned.

These facts, together with the testimony of ancient geographers which

scarcely admits of any other explanation, are thought to prove that all

the waters of the Upper Arno were originally discharged through the Val

di Chiana into the Tiber, and that a part of them still continued to

flow, at least occasionally, in that direction down to the days of the

Roman empire, and perhaps for some time later. The depression of the bed

of the Arno, and the raising of that of the valley by the deposits of

the lateral torrents, finally cut off the branch of the river which had

flowed to the Tiber, and all its waters were turned into its present

channel, though the drainage of the principal part of the Val di Chiana

appears to have been in a south-eastwardly direction until within a

comparatively recent period.

In the sixteenth century the elevation of the bed of the valley had

become so considerable, that in 1551, at a point about ten miles south

of the Arno, it was found to be not less than one hundred and thirty

feet above that river; then followed a level of ten miles, and then a

continuous descent to the Paglia. Along the level portion of the valley

was a boatable channel, and lakes, sometimes a mile or even two miles in

breadth, had formed at various points farther south. At this period the

drainage of the summit level might easily have been determined in either

direction, and the opposite descents of the valley made to culminate at

the north or at the south end of the level. In the former case, the

watershed would have been ten miles south of the Arno; in the latter,

twenty miles, and the division of the valley into two opposite slopes

would have been not very unequal.

Various schemes were suggested at this time for drawing off the stagnant

waters, as well as for the future regular drainage of the valley, and

small operations for those purposes were undertaken with partial

success; but it was feared that the discharge of the accumulated waters

into the Tiber would produce a dangerous inundation, while the diversion

of the drainage into the Arno would increase the violence of the floods

to which that river was very subject, and no decisive steps were taken.

In 1606 an engineer, whose name has not been preserved, proposed, as the

only possible method of improvement, the piercing of a tunnel through

the hills bounding the valley on the west to convey its waters to the

Ombrone, but the expense and other objections prevented the adoption of

this scheme. [Footnote: Morozzi, Dello stato dell' Arno, ii., pp. 39,

40.] The fears of the Roman Government for the safety of the basin of

the Tiber had induced it to construct embankments across the portion of

the valley lying within its territory, and these obstructions, though

not specifically intended for that purpose, naturally promoted the

deposit of sediment and the elevation of the bed of the valley in their

neighborhood. The effect of this measure and of the continued

spontaneous action of the torrents was, that the northern slope, which

in 1551 had commenced at the distance of ten miles from the Arno, was

found in 1605 to begin nearly thirty miles south of that river, and in

1645 it had been removed about six miles farther in the same direction.

[Footnote: Morozzi, Dello stato, etc., dell' Arno, ii., pp. 39, 40.]

In the seventeenth century the Tuscan and Papal Governments consulted

Galileo, Torricelli, Castelli, Cassini, Viviani, and other distinguished

philosophers and engineers, on the possibility of reclaiming the valley

by a regular artificial drainage. Most of these eminent physicists were

of opinion that the measure was impracticable, though not altogether for

the same reasons; but they seem to have agreed in thinking that the

opening of such channels, in either direction, as would give the current

a flow sufficiently rapid to drain the lands properly, would dangerously

augment the inundations of the river--whether the Tiber or the

Arno--into which the waters should be turned. The general improvement of

the valley was now for a long time abandoned, and the waters were

allowed to spread and stagnate until carried off by partial drainage,

infiltration, and evaporation. Torricelli had contended that the slope

of a large part of the valley was too small to allow it to be drained by

ordinary methods, and that no practicable depth and width of canal would

suffice for that purpose. It could be laid dry, he thought, only by

converting its surface into an inclined plane, and he suggested that

this might be accomplished by controlling the flow of the numerous

torrents which pour into it, so as to force them to deposit their

sediment at the pleasure of the engineer, and, consequently, to elevate

the level of the area over which it should be spread. [Footnote:

Torricelli thus expressed himself on this point: "If we content

ourselves with what nature has made practicable to human industry, we

shall endeavor to control, as far as possible, the outlets of these

streams, which, by raising the bed of the valley with their deposits,

will realize the fable of the Tagus and the Pactolus, and truly roll

golden sands for him that is wise enough to avail himself of

them."--Fossombroni, Memoris sopra la Val di China, p. 219.] This plan

did not meet with immediate general acceptance, but it was soon adopted

for local purposes at some points in the southern part of the valley,

and it gradually grew in public favor and was extended in application

until its final triumph a hundred years later.

In spite of these encouraging successes, however, the fear of danger to

the valley of the Arno and the Tiber, and the difficulty of an agreement

between Tuscany and Rome--the boundary between which states crossed the

Val di Chiana not far from the half-way point between the two

rivers--and of reconciling other conflicting interests, prevented the

resumption of the projects for the general drainage of the valley until

after the middle of the eighteenth century. In the meantime the science

of hydraulics had become better understood, and the establishment of the

natural law according to which the velocity of a current of water, and

of course the proportional quantity discharged by it in a given time,

are increased by increasing its mass, had diminished if not dissipated

the fear of exposing the banks of the Arno to greater danger from

inundations by draining the Val di China into it. The suggestion of

Torricelli was finally adopted as the basis of a comprehensive system of

improvement, and it was decided to continue and extend the inversion of

the original flow of the waters, and to turn them into the Arno from a

point as far to the south as should be found practicable. The conduct of

the works was committed to a succession of able engineers who, for a

long series of years, were under the general direction of the celebrated

philosopher and statesman Fossombroni, and the success has fully

justified the expectations of the most sanguine advocates of the scheme.

The plan of improvement embraced two branches: the one, the removal of

obstructions in the bed of the Arno, and, consequently, the further

depression of the channel of that river, in certain places, with the

view of increasing the rapidity of its current; the other, the gradual

filling up of the ponds and swamps, and raising of the lower grounds of

the Val di Chiana, by directing to convenient points the flow of the

streams which pour down into it, and there confining their waters by

temporary dams until the sediment was deposited where it was needed. The

economical result of these operations has been, that in 1835 an area of

more than four hundred and fifty square miles of pond, marsh, and damp,

sickly low grounds had been converted into fertile, healthy, and

well-drained soil, and, consequently, that so much territory has been

added to the agricultural domain of Tuscany. But in our present view of

the subject, the geographical revolution which has been accomplished is

still more interesting. The climatic influence of the elevation and

draining of the soil must have been considerable, though I do not know

that an increase or a diminution of the mean temperature or

precipitation in the valley has been established by meteorological

observation. There is, however, in the improvement of the sanitary

condition of the Val di Chiana, which was formerly extremely unhealthy,

satisfactory proof of a beneficial climatic change. The fevers, which

not only decimated the population of the low grounds but infested the

adjacent hills, have ceased their ravages, and are now not more frequent

than in other parts of Tuscany. The strictly topographical effect of the

operations in question, besides the conversion of marsh into dry

surface, has been the inversion of the inclination of the valley for a

distance of thirty-five miles, so that this great plain which, within a

comparatively short period, sloped and drained its waters to the south,

now inclines and sends its drainage to the north. The reversal of the

currents of the valley has added to the Arno a new tributary equal to

the largest of its former affluents, and a most important circumstance

connected with this latter fact is, that the increase of the volume of

its waters has accelerated their velocity in a still greater proportion,

and, instead of augmenting the danger from its inundations, has almost

wholly obviated that source of apprehension. [Footnote: Arrian observes

that at the junction of the Hydaspes and the Acesines, both of which are

described as wide streams, "one very narrow river is formed of two

confluents, and its current is very swift."--Arrian, Alex. Anab., vi.,

4.