THE LIFE HISTORIES OF RIVERS
=The intricate pattern of river etchings.=—The attack of the weather
upon the solid lithosphere destroys the integrity of its surface
layer, and through reducing it to rock débris makes it the natural
prey of any agent competent to carry it along the surface. We have
seen how, for short distances, gravity unaided may pile up the débris
in accumulations of talus, and how, when assisted by thaw water which
has soaked into the material, it may accomplish a slow migration by a
peculiar type of soil flow. Yet far more potent transporting agencies
are at work, and of these the one of first importance is running
water. Only in the hearts of great deserts or in the equally remote
white deserts of the polar regions is the sound of its murmurings
never heard. Every other part of the earth’s surface has at some time
its running water coursing in valleys which it has itself etched into
the surface. It is this etching out of the continents in an intricate
pattern of anastomosing valleys which constitutes the chief difference
between the land surface and the relatively even floor of the oceans.
=The motive power of rivers.=—Every river is born in throes of Mother
Earth by which the land is uplifted and left at a higher level than
it was before. It is the difference of elevation thus brought about
between separated portions of the land areas that makes it possible for
the water which falls upon the higher portions to descend by gravity to
the lower. This natural “head” due to differences of elevation is the
motive power of the local streams, and for each increase in elevation
there is an immediate response in renewed vigor of the streams. The
elevated area off which the rivers flow is here termed an upland.
The velocity of a stream will be dependent not only upon the difference
in altitude between its source and its mouth, but upon the distance
which separates them, since this will determine the grade. The level
of the mouth being the lowest which the stream can reach is termed
the _base level_, and the current is fixed by the slope or declivity.
The capacity to lift and transport rock débris is augmented at a quite
surprising rate with every increase in current velocity, the law being
that the weight of the heaviest transportable fragment varies with the
sixth power of the velocity of the current. Thus if one stream flows
twice as rapidly as another, it can transport fragments which are
sixty-four times as heavy.
=Old land and new land.=—The uplifts of the continents may proceed
without changes in the position of the shore lines, in which case
areas, already carved by streams but no longer actively modified by
them, are worked upon by tools freshly sharpened and driven by greater
power. The land thus subjected to active stream cutting is described
as old land, and has already had engraved upon it the characteristic
pattern of river etchings, albeit the design has been in part effaced.
If, upon the other hand, the shore line migrates seaward with the
uplift, a portion of the relatively even sea floor, or new land, is
elevated and laid under the action of the running water. As we are to
see, stream cutting is to some extent modified when a river pattern is
inherited from the uplift. The uplift, whether of old land only or of
both old land and new land, marks the starting point of a new river
history, usually described as an _erosion cycle_.
[Illustration: FIG. 165.—Two successive forms of gullies from the
earliest stage of a river’s life (after Salisbury and Atwood).]
=The earlier aspects of rivers.=—Though geologists have sometimes
regarded the uplift of the continents as a sort of upwarping in a
continuous curved surface, the discussions of river histories and
the pictorial illustrations of them have alike clearly assumed that
the uplift has been essentially in blocks and that the elevated area
meets the lower lying country or the sea in a more or less definite
escarpment. The first rivers to develop after the uplift may be
described as gullies shaped by the sudden downrush of storm waters and
spaced more or less regularly along the margin of the escarpment (Fig.
165). These gullies are relatively short, straight, and steep; they
have precipitous walls and few, if any, tributaries.
[Illustration: FIG. 166.—Partially dissected upland (after Salisbury
and Atwood).]
With time the gully heads advance into the upland as they take on
tributaries; and so at length they in part invest it and dissect it
into numerous irregularly bounded and flat-topped tables which are
separated by cañons (Fig. 166). At the same time the grade of the
channel is becoming flatter, and its precipitous walls are being
replaced by curving slopes, as will be more fully described in the
sequel. It is because of this progressive reduction of grades with
increasing age that the early stages of a river’s life are much the
most turbulent of its history. The water then rushes down the steep
grades in rapids, and is often at times opened out in some basin to
form a lake where differences of uplift have been characteristic of
neighboring sections. For several reasons such basins in the course of
a stream are relatively short lived (Chapter XXX), and they disappear
with the earlier stages of the river history.
=The meshes of the river network.=—From the continued throwing out
of new tributaries by the streams, the meshes in the river network
draw more closely together as the stages of its history advance. The
closeness of texture which is at last developed upon the upland is in
part determined by the quantity of rainfall, so that in New Jersey with
heavy annual precipitation the meshes in the network are much smaller
than they are, for example, upon the semiarid or arid plains of the
western United States. Its design will, however, in either case more
or less clearly express the plan of rock architecture which is hidden
beneath the surface (Chapter XVII).
=The upper and lower reaches of a river contrasted.=—From the fact
that the river progressively invades new portions of the upland and
lays the acquired sections under more and more thorough investment, it
has near its headwaters for a long time a frontier district which may
be regarded as youthful even though the sections near its mouth have
reached a somewhat advanced stage. The newly acquired sections of river
valley may thus possess the steep grade and precipitous walls which are
characteristic of early gullies and cañons and are in contrast with the
more rounded and flat-bottomed sections below. Lateral streams, from
the fact that they are newer than the main or trunk stream to which
they are tributary, likewise descend upon somewhat steeper grades (Fig.
167).
[Illustration: FIG. 167.—Characteristic longitudinal sections of the
upper portion of a river valley and its tributaries (after scaled
sections by Nussbaum).]
=The balance between degradation and aggradation.=—We have seen
that the power to transport rock fragments is augmented at a most
surprising rate with every increase in the current velocity. While the
lighter particles of rock may be carried as high up as the surface of
the water, the heavier ones are moved forward upon the bottom with
a combined rolling and hopping motion aided by local eddies. Those
particles which come in contact with the bottom or sides of the
channel abrade its surface so as ever to deepen and widen the valley.
This cutting accomplished by partially suspended débris in rapidly
moving currents of water is known as _corrasion_ and the stream is said
to be _incising_ its valley.
As the current is checked upon the lower and flatter grades, some of
its load of sediment, and especially the coarser portion, will be
deposited and so partially fill in the channel. A nice balance is thus
established between _degradation_ and the contrasted process known
as _aggradation_. The older the river valley the flatter become the
grades at any section of its course, and thus the point which separates
the lower zone of aggradation from the upper one of degradation moves
steadily upstream with the lapse of time.
=The accordance of tributary valleys.=—It is a consequence of the
great sensitiveness of stream corrasion to current velocity that
no side stream may enter the trunk valley at a level above that
of the main stream—the tributary streams enter the trunk stream
_accordantly_. Each has carved its own valley, and any abrupt increase
in gradient of the side streams near where they enter the main stream
would have increased the local corrasion at an accelerated rate and so
have cut down the channel to the level of the trunk stream.
=The grading of the flood plain.=—All rivers are subject to seasonal
variations in the volume of their waters. Where there are wet and
dry seasons these differences are greatest, and for a large part of
the year the valleys in such regions may be empty of water, and are
in fact often utilized for thoroughfares. In the temperate climates
of middle latitudes rivers are generally flooded in the spring when
the winter snows are melted, though they may dwindle to comparatively
small streams during the late summer. In the upper reaches of the
river the current velocities are such that the usual river channel may
carry all the water of flood time; but lower down and in the zone of
aggradation, where the current has been checked, the level of the water
rises in flood above the banks of its usual channel and spreads over
the surrounding lowlands. As a deposit of sediment is spread upon the
surface, the succession of the annual deposits from this source raises
the general level as a broad floor described as the _flood plain_ of
the river.
=The cycles of stream meanders.=—The annual flooding with water
and simultaneous deposition of silt is not, however, the only
grading process which is in operation upon the flood plain. It is
characteristic of swift currents that their course is maintained
in relatively straight lines because of the inertia of the rapidly
moving water. In proportion as their currents become sluggish, rivers
are turned aside by the smallest of obstructions; and once diverted
from their straight course, a law of nature becomes operative which
increases the curvature of the stream at an accelerated rate up to
a critical point, when by a change, sudden and catastrophic, a new
and direct course is taken, to be in its turn carried through a
similar cycle of changes. This so-called _meandering_ of a stream is
accompanied by a transfer of sediment from one bend or meander of the
river to those below and from one bank to the other. Inasmuch as the
later meanders cross the earlier ones and in time occupy all portions
of the plain to the same average extent, a process of rough grading is
accomplished to which the annual overflow deposit is supplementary.
[Illustration:
FIG. 168.—Map and sections of a stream meander. The course of the main
current is indicated by the dashed line.]
The course of the current in consecutive meanders and the cross
sections of the channel which result directly from the meandering
process will be made clear from examination of Fig. 168. So soon
as diverted from its direct course, the current, by its inertia of
motion, is thrown against the outer or convex side so as to scour or
corrade that bank. Upon the concave or inner side of the curve there
is in consequence an area of slack water, and here the silt scoured
from higher meanders is deposited. The scouring of the current upon
the outer bank and the filling upon the inner thus gives to the cross
section of the stream a generally unsymmetrical character (Fig. 168
_ab_). Between meanders near the point of inflection of the curve, and
there only, the current is centered in the middle of the channel and
the cross section is symmetrical (Fig. 168 _cd_).
[Illustration: FIG. 169.—Tree in part undermined upon the outer bank
of a meander.]
The scour upon the convex side of a meander causes the river to swing
ever farther in that direction, and through invasion of the silted
flood plain to migrate across it. Trees which lie in its path are
undermined and fall outward in the stream with tops directed with the
current (Fig. 169). Whenever the flood plain is forested, the fallen
trees may be so numerous as to lie in ranks along the shore, and at the
time of the next flood they are carried downstream to jam in narrow
places along the channel and give the erroneous impression that the
flood has itself uprooted a section of forest (see p. 418).
[Illustration:
FIG. 170.—Diagrams to show the successive positions of stream meanders
and the relatively stationary point near the sharpest curvature.]
=The cut-off of the meander.=—As the meander swings toward its
extreme position it becomes more and more closely looped. Adjacent
loops thus approach nearer and nearer to each other, but in the
successive positions a nearly stationary point is established near
where the river makes its sharpest turn (Fig. 170, _G_, and Fig. 454,
p. 417). At length the neck of land which separates meanders is so
narrow that in the next freshet a temporary jamming of logs within the
channel may direct the waters across the neck, and once started in
the new direction a channel is scoured out in the soft silt. Thus by
a breaking through of the bank of the stream, a so-called “crevasse”,
the river suddenly straightens its course, though up to this time it
has steadily become more and more sharply serpentine. After the cut-off
has occurred, the old channel may for a time continue to be used by the
stream in common with the new one, but the advantage in velocity of
current being with the cut-off, the old channel contains slacker water
and so begins to fill with silt both at the beginning and the end of
the loop. Eventually closed up at both ends, this loop or “ox-bow”
is entirely separated from the new channel, and once abandoned of the
stream is transformed into an ox-bow lake (Fig. 171 and p. 415).
[Illustration: FIG. 171.—An ox-bow lake in the flood plain of a river.]
=Meander scars.=—Swinging as it occasionally does in its meanderings
quite across the flood plain and against the bank of the earlier
degrading river in this section, the meander at times scours the high
bank which bounds the flood plain, and undermining it in the same
manner, it excavates a recess of amphitheatral form which is known as
a meander _scar_ (Fig. 172). At length the entire bank is scarred in
this manner so as to present to the stream a series of concave scallops
separated by sharp intermediate salients of cuspate form.
[Illustration:
FIG. 172.—Schematic representation of a series of river terraces. _a_,
_b_, _c_, _e_, successive terraces in order of age. _d_, _d_, _d_, _d_,
terrace slopes formed of meander scars.]
=River terraces.=—Whenever the river’s history is interrupted by a
small uplift, or the base level is for any reason lowered, the stream
at once begins to sink its channel into the flood plain. Once more
flowing upon a low grade, it again meanders, and so produces new walls
at a lower level, but formed, like the first, of intersecting meander
scars. Thus there is produced a new flood plain with cliff and terrace
above, which is known as a _river terrace_. A succession of uplifts or
of depressions of the base level yields terraces in series, as they
appear schematically represented in Fig. 172. Such terraces are to be
found well developed upon most of our larger rivers to the northward of
the Ohio and Missouri. The highest terrace is obviously the remnant of
the earliest flood plain, as the lowest represents the latest.
=The delta of the river.=—As it approaches its mouth the river moves
more and more sluggishly over the flat grades, and swings in broader
meanders as it flows. Yet it still carries a quantity of silt which is
only laid down after its current has been stopped on meeting the body
of standing water into which it discharges. If this be the ocean, the
salinity of the sea water greatly aids in a quick precipitation of the
finest material. This clarifying effect upon the water of the dissolved
salt may be strikingly illustrated by taking two similar jars, the
one filled with fresh and the other with salt water, and stirring the
same quantity of fine clay into each. The clay in the salt water is
deposited and the water cleared long before the murkiness of the other
has disappeared.
By the laying down of the residue of its burden of sediment where it
meets the sea, the river builds up vast plains of silt and clay which
are known as deltas and which often form large local extensions of the
continents into the sea. Whereas in its upper reaches the river with
its tributary streams appears in the plan like a tree and its branches,
in the delta region the stream, by dividing into diverging channels
called distributaries (Fig. 458, p. 420), completes the resemblance to
the tree by adding the roots. From the divergence of the distributaries
upon the delta plain the Greek capital letter Δ is suggested and has
supplied the name for these deposits. Of great fertility, the delta
plains of rivers have become the densely populated regions of the
globe, among which it is necessary to mention only the delta of the
Nile in Egypt, those of the Ganges and Brahmaputra in India, and those
of the Hoang and Yangtse rivers in China.
=The levee.=—When the snows thaw upon the mountains at the headwaters
of large rivers, freshets result and the delta regions are flooded. At
such times heavily charged with sediment, a thin deposit of fertile
soil is left upon the surface of the delta plain, and in Egypt
particularly this is depended upon for the annual enrichment of the
cultivated fields. Though at this time the waters spread broadly over
the plain, the current still continues to flow largely within the
normal channel, so that the slack water upon either side becomes the
locus for the main deposit of the sediment. There is thus built up on
either side of the channel a ridge of silt which is known as a _levee_,
and this bank is steadily increased in height from year to year (Fig.
452).
To prevent the danger of floods upon the inhabited plains, artificial
levees are usually raised upon the natural ones, and in a country like
Holland, such levees (dikes) involve a large expenditure of money and
no small degree of engineering skill and experience to construct. So
important to the life of the nation is the proper management of its
dikes, that in the past history of China each weak administration has
been marked by the development of graft in this important department
and by floods which have destroyed the lives of hundreds of thousands
of people.
[Illustration:
FIG. 173.—“Bird-foot” delta of the Mississippi River.]
Wherever there has been a markedly rapid sinking upon a delta region,
and depressions are common in delta territory, no doubt as a result of
the loading down of the crust, the river may present the paradoxical
condition of flowing at a higher level than the surrounding country.
Between the levees of neighboring distributaries there are peculiar
saucer-shaped depressions of the country which easily become filled
with water. At the extremity of the delta the levee may be the only
land which shows above the ocean surface, and so present the peculiar
“bird-foot” outline which is characteristic of the extremity of the
Mississippi delta, though other processes than the mere sinking of the
deposits may contribute to this result (Fig. 173).
=The sections of delta deposits.=—If now we leave the plan of the
delta to consider the section of its deposits, we find them so
characteristic as to be easily recognized. Considered broadly, the
delta advances seaward after the manner of a railroad embankment
which is being carried across a lake. Though the greater portion of
the deposit is unloaded upon a steep slope at the front, a smaller
amount of material is dropped along the way, and a layer of extremely
fine material settles in advance as the water clears of its finely
suspended particles (Fig. 174). Simultaneous deposits within a delta
thus comprise a nearly horizontal layer of coarser materials, the
so-called top-set bed; the bulk of the deposit in a forward sloping
layer, the so-called fore-set bed; and a thin film of clay which is
extended far in advance, the bottom-set bed (Fig. 174, 2). If at any
point a vertical section is made through the deposits, beds deposited
in different periods are encountered; the oldest at the bottom in a
horizontal position, the next younger above them and with forward
dip, and the youngest and coarsest upon the top in nearly horizontal
position (Fig. 174, 3).
[Illustration:
FIG. 174.—Diagrams to show the nature of delta deposits as exhibited
in section.]
It has been estimated that the surface of the United States is now
being pared down by erosion at the average rate of an inch in 760
years. The derived material is being deposited in the flood plain
and delta regions of its principal rivers. Some 513 million tons of
suspended matter is in the United States carried to tidewater each
year, and about half as much more goes out to sea as dissolved matter.
If this material were removed from the Panama Canal cutting, an 85-foot
sea-level canal would be excavated in about 73 days. The Mississippi
River alone carries annually to the sea 340 million tons of suspended
matter, or two thirds of the entire amount removed from the area of the
United States as a whole. It is thus little wonder that great deltas
have extended their boundaries so rapidly and that the crust is so
generally sinking beneath the load.