The valley of the Lower Mississippi, 330,000
_______
Total area, 1,210,000
As a natural consequence of the drainage of this immense area, the
Mississippi receives into its waters a large amount of suspended earthy
matter. This, however, does not very strikingly appear on the upper
river, its own banks and those of its tributaries being more of a
gravelly character and less friable than lower down. The gravity of
particles, therefore, worn from the bed and sides of the channel above,
unless the current be exceedingly strong, is greater than the buoyant
capacity of the water, and falls to the bottom, along which, sometimes,
it is forced by the abrasion of the water, until it meets some
obstruction, which gathers the particles into shoal formations. This
fact causes much inconvenience in the navigation of the upper rivers.
It is not until we reach the confluence of the streams of Southern
Illinois and Missouri, that the sediment of the river becomes striking.
Those streams, freighted with the rich loam and vegetable matter of the
prairies of the east and west, soon change entirely the appearance of
the Mississippi. Above the Missouri, the river is but slightly tinged;
and indeed, after that great current enters, for some distance the two
run side by side in the same channel, and yet are divided by a very
distinct line of demarcation. It is only after the frequent sinuosities
of the channel, that the two waters are thrown into each other and
fairly blend. The sedimentary condition of the Missouri is so great that
drift floating upon its muddy surface, by accretion becomes so heavily
laden with earthy matter that it sinks to the bottom. This precipitation
of drift has taken place to such an extent, that the bed of the Missouri
is in many places completely covered to a great depth by immense fields
of logs. Of all the silt thrown into the Mississippi, the Missouri
furnishes about one third.
After receiving the Missouri, next enters the Ohio. The water of this
river is less impregnated than the Missouri, though not by any means
free from silt. The country through which it flows is mountainous, and
the soil hard, and does not afford the same facility of abrasive action
as that of the other rivers.
From the mouth of the Ohio, the Mississippi pursues a course of nearly
four hundred miles, when it receives the turbid waters of the White and
Arkansas Rivers. In the intervening distance a large number of small
currents, more or less largely sedimentary, according to the character
of the country through which they run, enter the Mississippi, in the
aggregate adding materially to the sediment of the receiving stream. The
White and Arkansas carry in their waters a large amount of
unprecipitated matter. In this vicinity, too, sets in that singular
system of natural safeguards of the surrounding country, the bayous. The
country here also changes its appearance, becoming flat and swampy, and
in some parts attaining but a few feet above the flood of the river,
whereas in other parts, as we approach the Gulf, the country is even
lower than the river.
The miasmatic and poisonous water of the Yazoo next enters, about ten
miles above Vicksburg. This river is more deeply impregnated with a
certain kind of impurities than any other tributary of the Mississippi.
The waters are green and slimy, and almost sticky with vegetable and
animal decomposition. During the hot season the water is certain
disease, if taken into the stomach. The name is of Indian origin, and
signifies 'River of Death.' The Yazoo receives its supply from bayous
and swamps, though it has several considerable tributaries.
Below the Yazoo, on the west side, enters the Red. The name indicates
the peculiar caste of its water. This river carries with it the washings
of an extensive area of prairies and swamps, and is the last of the
great tributaries. Hence the tendency of streams is directly to the
Gulf, and that network of lateral branches, of which we will hereafter
speak, begins.
We have only considered the most prominent tributaries: the sediment
also brought down by the numerous smaller streams is very great, and
makes great additions to the immense buoyant matter of the Mississippi.
The river itself from its own banks scours the larger portion of the
sediment it contains; and in so gigantic a scale is this carried on,
that it can be seen without the exercise of any very remarkable powers
of sight. It is not by the imperceptible degrees usually at work in
other streams, but often involves in its execution many acres of
adjoining land. It will be interesting to consider this more fully.
By a curious freak of nature, the tendency of the channel of the
Mississippi is always toward one or the other of its banks, being
influenced by the direction of its bends. The principle is one of nicely
regulated refraction. If the river were perfectly straight, the gravity
and inertia of its waters would move in a right line, with a velocity
beyond all control. But we find the river very sinuous, and the momentum
of current consequently lessened. For example, striking in an arm of the
river, by the inertia of the moving volume, the water is thrown, and
with less velocity, upon the opposite bank, which it pursues until it
meets another repellent obstacle, from which it refracts, taking
direction again for the other side. Above the Missouri, the river is
principally directed by the natural trough of the valley. Below this,
however, the channel is purely the work of the river itself, shaped
according to the necessities of sudden changes or obstructions. This is
proven by the large number of old and dry beds of the river frequently
met with, the channel having been diverted in a new direction by the
accumulation of sediment and drift which it had not the momentum to
force out.
Where the gravity of the greatest volume and momentum of water falls
upon the bed of the river, there is described the thread of the channel,
and all submerged space outside of this, though in the river, acts as a
kind of reservoir, where eddies the surplus water until taken up by the
current. And it always happens, where the channel takes one bend of the
bed, a corresponding tongue of shallow water faces the indenture. Where
the river, by some inexplicable cause, has been thrown from its regular
channel, or its volume of water embarrassed by some difficulties along
the banks, the effect is immediately perceived upon the neighboring
bank. The column of water thus impinged against it at once acts upon the
bank, and, singularly enough, exerts its strongest abrasive action at
the bottom, undermining the bank, which soon gives way, and instead of
toppling forward, it noiselessly slides beneath the water and
disappears. Acres of land have thus been carried away in an incredibly
short time, and without the slightest disruption of the serene flow of
the mighty current.
This carrying away of the banks, immense as is the amount of earth
thrown into the waters of the river, has no sensible effect in blocking
or directing the current, though it imperceptibly raises the channel.
The force of the water does not permit its entire settlement in
quantities at any one place, but distributes it along the bottom and
shores below. Were this not the case, it is easily to be seen, the
abrasion of the river banks would be greatly increased, and the
destruction of the bordering lands immense.
A singular feature resulting from the above may here be mentioned. By
pursuing the course of the river, a short distance below, on the
opposite bank, it will be seen that a large quantity of the earth
introduced into the current by the falling of the banks, has been thrown
up in large masses, forming new land, which, in a few seasons, becomes
arable. That which is not thus deposited, as already stated, is
transported below, dropping here and there on the way, until what is
left reaches the Gulf, and is precipitated upon the 'bars' and 'delta,'
at the mouth. It not unfrequently happens that planters along the river
find themselves suddenly deprived of some of their acres, while one
almost opposite finds himself as unexpectedly blessed with a bountiful
increase of his domain.
From causes almost similar to those given to explain the sudden and
disastrous changes of the channel of the river, are also produced those
singular shortenings, known as 'cut-offs,' which are so frequently met
with on the Mississippi. At a certain point the force of the current is
turned out of its path and impinged against a neck of land, that has,
after years of resistance, been worn down to an exceedingly small
breadth. Possibly the river has merely worn an arm in its side, leaving
an extensive bulge standing out in the river, and connected with the
mainland by an isthmus. The river striking in this arm, and not having
sufficient scope to rebound toward the other bank, is thrown into a
rotary motion, forming almost a whirlpool. The action of this motion
upon the banks soon reduces the connecting neck, which separates and
blocks the waters, until, at last, no longer able to cope with the great
weight resting against it, it gives way, and the river divides itself
between this new and the old channel.
Nor do these remarkable instances of abrasive action constitute the
entire washing from the banks. The whole length of the river is subject
to a continual deposit and taking up of the silt, according to the
buoyant capacity of the water. This, too, is so well regulated that the
quantity of earthy matter held in solution is very nearly the same,
being proportioned to the force of the current. For instance, if the
river receive more earth than it can sustain, the surplus sediment
drops upon the bottom or is forced up upon the sides. If the river be
subject to a rise, a proportionate quantity of the dropped sediment is
again taken up, and carried along or deposited again, according to the
capacity of the water. By this means a well-established average of silt
is at all times found buoyant in the river.
Having briefly examined the sedimentary character of the Mississippi,
some investigations as to the proportion of sediment to water may be of
interest. And it is well to state here that a mean stage of flow is
taken as the basis upon which to start the experiments. The experiments
and analysis of the water were made by Professor Riddell, at intervals
of three days, from May 21st to August 13, 1846, and reported to the
Association of American Geologists and Naturalists.
The water was taken in a pail from the river in front of the city of New
Orleans, where the current is rather swift. That portion of the river
contains a fair average of sedimentary matter, and it is sufficiently
distant from the _embouchure_ of the last principal tributary to allow
its water to mix well with that of the Mississippi.
'The temperature,' says the Professor, 'was observed at the time,
and the height of the river determined. Some minutes after, the
pail of water was agitated, and two samples of one pint each
measured out. The measure graduated by weighing at 60 degrees
Fahrenheit 7,295.581 grains of distilled water. After standing a
day or two, the matter mechanically suspended would subside to the
bottom. Nearly two thirds of the clear supernatant liquid was next
decanted, while the remaining water, along with the sediment, was
in each instance poured upon a double filter, the two parts of
which had previously been agitated, to be of equal weight. The
filters were numbered and laid aside, and ultimately dried in the
sunshine, under like circumstances, in two parcels, one embracing
the experiments from May 22 to July 15, the other from July 17 to
August 13. The difference in weight between the two parts of each
double filter was then carefully ascertained, and as to the inner
filter alone the sediment was attached, its excess of weight
indicated the amount of sediment.'
As the table may be interesting, showing the height and temperature of
the water as well as the result of the experiments at the different
times, we introduce it complete:
TABLE _showing the Quantity of Sediment contained in the Water
of the Mississippi River_.
Date of Height Temperature. Grains of Sediment
Experiment. of River in a Pint of
above Water.
Low
Water.
1846. ft. in. ° A. B.
May 21 10 11 72 6.66 7.00
" 25 10 11 73 9.08 9.12
" 27 10 10 78 7.80 9.00
" 29 11 0 74 7.30 8.10
June 2 11 1 75 4.80 5.45
" 4 11 1 75 7.87 6.10
" 6 11 4 75 4.60 4.90
" 8 11 4 75.5 5.48 5.60
" 10 10 4 76 6.70 6.80
" 12 10 8 76 6.50 6.30
" 14 10 5 76.5 6.00 6.00
" 16 10 4 76.5 6.47 6.15
" 20 10 4 77 7.08 7.40
" 22 10 2 77 9.88 9.00
" 24 9 8 77 8.40 8.48
" 26 8 9 77.5 8.25 8.78
" 28 8 0 79 9.10 9.58
July 1 7 2 79.5 9.15 9.25
July 3 7 2 79.5 9.63 10.00
" 6 6 2 81 8.20 7.57
" 8 6 0 81 7.30 6.96
" 10 6 1 81 6.12 6.28
" 13 5 9 82 7.72 7.30
" 15 5 10 82 6.67 6.60
" 17 5 10 82 4.45 4.57
" 20 5 4 82 6.07 5.75
" 24 3 10 84 5.76 5.72
" 27 3 1 84 4.77 4.60
" 29 3 11 84.5 4.28 4.13
Aug. 1 2 6 85 4.40 4.44
" 3 2 0 84 3.18 3.34
" 5 1 9 83 3.56 3.40
" 7 1 5 83 2.85 2.85
" 10 1 6 83 3.03 2.92
" 13 2 8 84 2.97 3.00
The mean average of column A. is 6.32.
" " " " " B. is 6.30.
Transcriber's note: This table has been reproduced exactly as in the
original.
'By comparison with distilled water,' says the same, 'the specific
gravity of the filtered river water we found to be 1.823; pint of such
water at 60° weighs 7,297.40.' Engineer Forehay says the sediment is 1
to 1,800 by weight, or 1 in 3,000 by volume.
Professor Riddell also comes to the following conclusions, after an
analytic investigation of the sediment. He took one hundred grains from
the river margin, dried it at 212° Fahrenheit, before weighing, and
found it to contain:
_Grains._
Silica, 74.15
Alumina, 9.14
Oxide of iron, 4.56
Lime, 2.08
Magnesia, 1.52
Manganese, 0.04
Potassa,} not determined
Soda, }
Phosphoric acid, 0.44
Sulphuric acid, 0.07
Carbonic acid, 0.74
Chlorine, 0.01
Water, 3.12
Organic matter, 3.10
-----
Total, 98.97
The existence of so large a quantity of sediment in the water of the
Mississippi, leads to divers formations in its bed. These formations are
principally 'bars' and 'battures.' The banks are also much affected.
When the water of the river, aided by the current, has attained its full
capacity of buoyant earth, as we have already said, the excess falls to
the bottom. Instead, however, of remaining permanently where it first
lodged, which would soon fill up the channel and cause the river to
overflow, the scouring of the water on the bottom forces a large portion
along with the current, though it be not suspended. Pursuing its course
for a while, some irregularity or obstruction falls in the way--a sunken
log, perhaps. This obstacle checks the progress of the moving earth--it
accumulates; the next wave brings down more--the accumulation becomes
greater; until, in the course of a few years, there is a vast field of
deposit, and a 'bar' is formed. These 'bars' often divert the channel,
and occasion the immense washings before alluded to.
Bars are generally found close to the banks, though there are examples
in which they extend in a transverse direction to the current. Bars of
this kind very much embarrass and endanger navigation in low water. At
Helena, Arkansas, there is an instance of a transverse bar, upon which,
in October, the water is less than six feet. These bars are formed of
sand, which seems to have been the heavier and less buoyant of the
components of the earth thrown into the current by abrasion, the lighter
portions having been separated by the water and carried off.
It will not be necessary to consider further the subject of bars in the
river, but those at its mouth deserve some attention. The subject is one
that has led to much theorizing, study, and fear--the latter
particularly, from an ill-founded supposition that they threaten to cut
off navigation into the Gulf.
Near its entrance into the Gulf, the Mississippi distributes its waters
through five outlets, termed passes, and consequently has as many
mouths. These are termed Pass à l'Outre, Northeast, Southeast, South,
and Southwest. They differ in length, ranging from three to nine miles.
They also all afford sufficient depth of water for commercial purposes,
except at their mouths, which are obstructed by bars. The depth of water
upon one of these is sufficient to pass large vessels; a second, vessels
of less size; and the rest are not navigable at all, as regards
sea-going vessels. These bars, too, are continually changing, according
to the winds or the currents of the river. It is a rather singular fact
that when one of the navigable passes becomes blocked, the river is
certain to force a channel of navigable depth through one of the
others, previously not in use; so that at no one time are all the passes
closed.
In looking into the past, and noticing the changes, it is recorded that
in 1720, of all the passes the South Pass was the only one navigable. In
1730, there was a depth of from twelve to fifteen feet, according to the
winds, and at another time even seventeen feet was known. In 1804, upon
the statement of Major Stoddard, written at that date, the East Pass,
called the Balize, had then about seventeen feet of water on the bar,
and was the one usually navigated. The South Pass was formerly of equal
depth, but was then gradually filling up. (This pass, at present, 1864,
is not at all navigated.) The Southwest Pass had from eleven to twelve
feet of water. The Northeast and Southeast Passes were traversed only by
small craft. Since 1830 the Southwest Pass has been gaining depth. This
and Pass à l'Outre are now the only two out of the five of sufficient
depth to admit the crossing of the larger class of vessels. The former,
however, is the one in most general use. All the other passes, with the
exception of the two mentioned, have been abandoned.
In regard to the changes and numerous singular formations at the mouths
of the Mississippi, we give a statement made by William Talbot, for
twenty-five years a resident of the Balize. He says:
'The bars at the various passes change very often. The channel
sometimes changes two and three times in a season. Occasionally one
gale of wind will change the channel. The bars make to the seaward
every year. The Southwest Pass is now the main outlet used. It has
been so only for three years, as at that time there was as much
water in the Northeast Pass as in it. The Southeast Pass was the
main ship channel twenty years ago; there is only about six feet of
water in that pass now; and where it was deepest then, there are
only a few inches of water at this time. The visible shores of the
river have made out into the Gulf two or three miles within my
memory. Besides the deposits of mud and sand, which form the bars,
there frequently rise up bumps, or mounds, near the channel, which
divert its course. These bumps are supposed to be the production of
salt springs, and sometimes are formed in a very few days. They
sometimes rise four or five feet above the surface of the water.'
He 'knew one instance when some bricks, that were thrown overboard
from a vessel outside the bar, in three fathoms of water, were
raised above the surface by one of these banks, and were taken to
the Balize, and used in building chimneys. In another instance, an
anchor, which was lost from a vessel, was lifted out of the water,
so that it was taken ashore. About twenty years ago, a sloop, used
as a lighter, was lost outside the bar in a gale of wind; several
years afterward she was raised by one of these strange formations,
and her cargo was taken out of her.'
We may say the bumps of which Mr. Talbot speaks are termed 'mud bumps,'
from the fact of being composed of sediment. They present a curious
spectacle as seen from a passing steamer. They are undoubtedly the
result of subterranean pressure, but from what cause, whether volcanic,
or the influence of the sea or river, or both, has not been determined.
Many speculations have been entered into in regard to these phenomena,
but as yet without fruitful result.
Leaving this digression, we proceed to notice that the theories set up
to explain the causes of the bars at the mouth of the river, have been
numerous and various. Some suppose them to be the result of the water of
the river meeting the opposing force of the Gulf waves, checking the
current, and causing a precipitation of the suspended sediment. Others
are of the opinion that the bars are entirely the effect of marine
action, and endeavor to show that the immense inward flow of the Gulf
washes up from its bed the vast accumulations that are continually
forming in the way of navigation.
After a personal observation and investigation, and as well after
frequent and free consultation with others, we are persuaded to
discredit the above-mentioned theories. The resistance of the Gulf does
not form the bars, though it exerts an influence. The immense volume and
force of water ejected from the river receives no immediate repellent
action from the Gulf, but extends into it many miles without the least
signs of disturbance, as may be plainly discovered even in the most
casual observation. It is known as well that the water of the river
remains perfectly palatable at a very close proximity to the sea. This
is a very good evidence of the superior force of the river's current.
The two volumes of water mix a considerable distance out at sea.
An able engineer states that, upon examination, he found a column of
fresh water seven feet deep and seven thousand feet wide, and discovered
salt water at eight feet below the surface. As the result of his
investigations, he divides the water into three strata, as follows:
1. Fresh water, running out at the top with a velocity of three miles an
hour.
2. Salt water, beneath the fresh, also running out at about the same
velocity.