Jacques W. (Jacques Wardlaw) Redway.

Elementary physical geography : an outline of physiography online

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the plain through which it flows de-
creases, the velocity of the current is
checked, and because of the slackening
current the water is constantly di'opping
its load of silt.'' Moreover, when the
latter has been dropped, the water can-
not pick it up again unless the current
is quickened, and must thereafter flow
around it.

Islands are common in rivers carry-
ing a considerable sediment. The an-
choring of a snag, or any other obstacle,
slackens the current and causes the de-
position of silt. The latter increases in
amount until finally it reaches to the sur-
face. Then vegetation gets root and an
island results.

The river which flows over a decreasing slope has a ten-
dency, therefore, to form loops in its lower course, and in
general the loops are long-lived. But when there is a
succession of years of increased volume of water, the con-
ditions are changed. Because the volume of water is
increased the current is quickened, and the water then
begins to pick up silt that it had previously dropped.
In time, the neck of the loop is cut away, and the river
shortens its channel — sometimes by twenty or thirty
miles.' The line of moats, or oxbow lakes, along the lower

Prairie du Chien






Mississippi marks the old loops and abauJoned channels
along this river. It is evident also that the great amount
of siU removed when a loop is destroyed must be carried

farther down stream and there
deposited. How would this
affect the river so far as the
formation of bars is con-
cerned ? As a matter of fact
the destruction of a loop is
attended by changes in the
chanuel that are noticeable
many miles both above and
below the loop ; and more
than a year elapsed after
Davis cut-off had formed be-
fore the changes ceased.
If, during a period of sev-
eral years, there is less than the usual rainfall, the stream
will probably increase the amplitude of its loops, and
even make new ones. With the coming of successive
years of greater rainfall, however, the volume of water is
increased, the current is quickened, and the water be-
gins to pick up and remove sediment that formerly it had
been unable to carry.

Growth and Development of Rivers. — A river and
its basin do not constitute a fixed, unchanging feature of
the land. On the contrary, every river passes through the
various stages of infancy, maturity', and old age ; and its
legitimate work is to carve away and remove its basin
until every part is worn away to base level. The moment
any plain or surface — such, for instance, as the coast plain
of New Jersey — is exposed to the action of the w^eather, the
water falling upon it begins to form channels * and flow to
the sea. Such a stream may be called an infant river.


At first the stream drains its water-shed very imperfect!}'.
It encounters many obstacles ; and if the slope is gentle, it
finds not a little difficulty in
making its channel. It is em-
barrassed by the inequalities ^hb legitimate work of a
of the surface, and because river

of them, lakes and swamps it removes the rock waste from a to b .■

v^i ^x ^ , i- A B, the old ; a' b , the new profile.

form in the slight depressions.

The channels are apt to be shallow and the divides between
the adjacent branches are neither permanent nor well de-
fined. In consequence, any unusual flood may result in
the abandonment of an old and the selection of a new
channel, Ked Eiver of the North, is an example of an in-
fant river.

As a stream reaches maturity its character is changed.
The channel is deepened and cut nearer to base level.
The gullies of the tributary
streams become ravines and
many of the latter are sculpt- ™fant stage of a river

ured into broad valleys. The t'" ""'"" '"" ^Z'^.l'i f ''""'"'' '" '*'
tributaries extend their chan-
nels backward and not infrequently capture the waters
of other streams less vigorous {See ill test ration, p. 1J3).
The mature stage is the age of its greatest vigor and power.
It may lengthen itself at both ends ; it may build a delta
at its mouth and extend the latter seaward, or it may cut
its headwater channels back- ^_^^^ s^ ^ -^ ^ - n
wards. '

The old age begins when '^^^ mature and senile stages


the river has cut away and rUe ma,;, stream .nd its tr.hutar.cs have

trnTltsnnrfpd nil flip nVlillblp earved deep channels in the plan, A B : In
iranspoiied ail IIIC avaiiaUlb a' f c' the remaining material has been

material within the reach of ""'''''' ''""^•

its various branches. Thereafter it can be revived only

by a gradual elevation of some part of its bed, by changes in



its slope, or by a considerable increase of its volume. Just
as a log moved against the saw results in cutting the tim-
ber, so a gradual uplift of the stream channel gives the
river fresh power and, for a time, rejuvenates it. An in-
crease in the volume, by quickening the current of the
stream, has also a similar effect. If, as in the case of the


The stream on tin- ri,^ht has uncovered the ledges of hard rock shozon in the margin and

falls have resulted.

uplift of Uinta Mountains across Green Eiver, the ele-
vation is long-continued, just so long will the river be
actively at work at that point. It can be rejuvenated
along its Avhole course by the uplift or tilting of its water-
shed in such a manner as to increase the current along the
whole extent. Uplift is nearly alwa^-s followed by exten-
sive stream corrosion.


Flood-Plains.— It often liapi^eus that a stream removes
more material from its upper or torrential part than it can
conveniently carry. The excess is then spread over the
middle and lower parts of the basin, forming the " bottom
lands " or Jfood-plain.

The deposition of sediment is the result of a slackening


The greater pari of the basin of each lias been removed. The tributary of the cen-
tral stream is carving- its way into the basin of the river on the right and will eventually
absorb the head waters of the latter.

of the current. In its infant stage the river has but lit-
tle cutting power and usually can carry all the material it
removes. When the headw^ater streams acquire greater
vigor, however, they remove so much rock waste that in
the middle and lower courses the water is overburdened
with it, and the process of flood plain-making begins.

Along that part of the plain occupied by the stream, un-
less the current is increased, the deposition of sediment is
constantly going on. The river builds its bed and banks
a little higher tlian the level on either side, continuing the


process until the comiiijj; of high water ; then it breaks
through its solf-inudo banks and selects a new channel in
lower land. By this process of adjustment, the river, in
turn, may occupy every part of its flood-plain. It there-
fore follows that flood-plains are due to the overburdening
of the current of the stream.

In its relation to life and its industries, the flood-plain is
the most important part of river physiography. The sur-
face is always level, making the region accessible to trans-
portation. Moreover, the rock waste is mixed with the ele-
ments that form the food of plant life, and therefore the
flood plain has a most fertile soil. In the Mississippi

The dark shading represents the sediment deposited by /loads.

Valley, for instance, Avhere the bluff lands produce twenty
bushels of wheat, the bottom lands yield thirty ; and if an
acre of bluif soil yields one bale of cotton, the same area
of bottom lands yields two. The greater part of the Chile
of geography is a simoom-swept desert with scarcely a sign
of life excepting that which pertains to the mines and the
mountain valleys. The real Chile is found in the densely-
peopled flood plains of the Andine streams. In these short
valleys are concentrated nearly all the activities that go
to make a great state.

Neither do we find the Egypt of history in the broad
stretch of land lying between the Red Sea and the Libyan
Desert. On the contrary, the four thousand years of his-
tory that has given to the world so much that goes to


make up modern civil izatiou, belongs to the flood plain
of the Nile. What has been the efiect of the Mesopo-
tamia on the history of the East?

Terraces. — After a river has cleared away all the rock-
waste and silt it can reach at the headwaters, the stream
may then turn its cutting- power against its flood-plain.
Instead of depositing sediment, the water begins to re-
move it. So it forms a deeper channel, along the sides of
which a new and lower flood-plain is built. The new flood
plain with the remnant of the old one form terraces. Of
these there ma}^ be three or even'four. Ultimately, how-
ever, nearly or quite all the flood-plain is removed.

Each marks a stage of down-ciitting. The darker shading shows the old bed of the river.

It is evident, therefore, that flood-plains and terraces are
merely incidents in the history of a river. Perhaps most
of the rivers of the United States are in the flood-plain
stage of their existence. Many of the streams of the north-
eastern ])art are in the terrace stage and are approaching
the pciriod of old age.

Deltas and Estuaries. — Salt water has a very remark-
able (dl'ect in chiuruig muddy, fresh water, and the mo-
ment the twf) mix the remaining silt held iii suspension
is quickly deposited. It follows, therefore, that, unless
the sediment is swept away by currents and tides, a



consiilorable accumulation will form at the moTith of the

The accompanying figure, the delta of the Mississippi
River, shows one of the most interesting types of delta for-
mation. It is evident, in this case at least, that the banks
of the delta are self-made, and that they have been formed
because the current has been checked more effectually at

,0 North Paea


South Paaa


the edges than in mid-stream. It is also evident that since
the lower Mississippi has occupied its present channel,
the river has built its lower part nearly one hundred miles
into the Gulf of Mexico.

The deltas of the Volga, and Ganges-Brahmaputra are
considerably more intricate than that of the Mississippi.
They are likewise older, and therefore more compactly


filled Avitli sediment. The delta of tlie Ganges-Brahmapu-
tra is perhaps the most extensive known. Its frontage on
the Indian Ocean is about two hundred miles, and its area
about twice that of the State of Texas. Much of the land
consists of shifting mud-flats, and the whole region is
subject to destructive inundations.

The delta of the Adige-Po has developed in a manner
not unlike that of the Ganges. Probably no other river of
its size in the world brings down more sediment than the
Po. As a result its delta is filling and extending very
rapidly — so rapidl}', in fact, that the town of Adria (Had-
ria), in Julius Csesar's time a seaport, is now more than
twenty miles inland. Ostia, in
early historic times, at the
mouth of the Tiber, is now
about seven miles inland.

With respect to economic
value, delta lands surpass al-
most all others in the possibil-
ities of productivit}'. The soil
is exceedingly rich, and, because
of the constant additions from
the river, it is enriched as fast
as its nutritive elements are
taken up by vegetation. The
Nile delta has long been known
as the granary of Egypt — the
Sunderbunds of the Ganges-
Brahmaputra are foremost
among the great rice-producing
fields of the world.

An inspection of any good
map will disclose the fact that while some rivers reach the
sea, each through a delta, others equally powerful with


/t part of a comparatively level plain
hai subsided below sea level.



respect to current, flow into estuaries. The Mississippi
and the Dehiware are contrasting examples. In the
former case the river has a tendency to block its mouth
with silt ; in the latter, a downward movement, or sink-
ing of the coast has practically drowned the mouth of
the river. Moreover, the action of the tide is usually
strong enough to keep the channel clear of silt between

Its situation adapts it for the centre of commerce of a newly-settled region.

bars. So, between the scouring action of the tide and
the sinking of the valley there is not only a broad, but
a deep area of water in most estuaries. If the mouth of
the river is in a coast plain, the estuary usually takes the
form similar to that of Delaware Eiver. Along a rugged
coast with an abrupt slope, however, the estuaries more
commonly are like the indentations of the Maine coast.
They are also numerous along the coast of Norway, where
they are cdiWe^ fjords.


lu rivers that flow 'Bto estuaries, the sediment is de-
posited in the form ol hars. In most instances two bars
are formed, one at tlie mouth, the other at the head of the
estuary. The reason for this double deposition of sediment
may be found in the action of the tides. Bars are formed
in comparatively still water, so, when the tide is slack at
flood, the deposition takes place at the head of the estuary
where the salt and the fresh water meet ; when the tide is
ebb, the two waters meet at the mouth of the estuary and
the deposition of sediment takes place at the lower end.

It is evident that the estuary favors commerce and navi-
gation while the delta on the whole is a hindrance. In
the case of the Mississippi, the navigable channel of the
delta has been kept open at an enormous expense. Of the
great seaports that are centres of commerce, by far the
greater number are on the shores of estuaries.

Cascades and Rapids, — In floAving to lower levels, if
the slope is abrupt, the water descends in a series of
rapids in the form of reaches more or less terraced. The
streams of the New England Plateau, and to a greater de-
gree the torrents of mountainous regions are illustrations.
In some instances, however, the stream plunges over a
vertical embankment in the form of a cascade or fall.
Of these, Niagara Falls, Spokane Falls, and those of the
Zambesi River are illustrations. In some instances moun-
tain streams make tremendous leaps. In the Yosemite
Valley, Merced River in three plunges falls 2,600 feet ; and
Bridal Veil fall, with a sheer pitch of 1,500 feet, reaches the
lower level in the form of fine water dust. TheStaubbach
("brook dust") of the Alps is a similar cascade, having a
fall of 900 feet. The Cascade Range of the United States
and the Lauterbrunnen ("nothing but fountains") of Al-
pine Europe are names that suggest the character of these



In some instances the stream has liad little to do with
making the cliffs over which it falls ; in other cases, how-
ever, the river itself has made the falls. If a stream flows
over the edge of a hard layer that rests on a softer material,
the latter will be more qnickly removed ; moreover, as the
softer layer is Avorn away, the fall becomes greater and the
water acquires an increased cutting power because it has a
constantly increasing distance to fall. Finally the stream

The siratiim at the lop of the fall is harder and more resistant than the strata below.

cuts away so much material at the lower level that a cata-
ract results.

In this manner the falls of Niagara Kiver were formed.
There is an upper layer of hard limestone surmounting a
deep layer of softer rock. The upper layer offers consid-
erable resistance to the water ; the lower layer is easily cut
away. Hence the falls are increasing rather than decreas-
ing in height. The upper layer, however, is worn not a


little, aucl the falls are receding up stream at the rate of
nearly two and oue-lialf feet a year.'**

There are many cataracts, however, that are the residt
of accident. Thus, a flow of lava across Columbia River
dammed the channel and formed the well-known cascades.
A similar lava flood at the same time obstructed its chief
tributar}^ the Willamette River, forunng the cataract at
Oregon City.

Falls and rapids frequently occur in the terrace stage
of rivers, although they may be developed in early matu-
rity. They are rarely found in the flood-plain age, because
the flood-plain buries all inequalities. After the stream
has carved away its flood-plain, it may uncover and develop
its former rapids and cascades.

Migration of Divides. — As a rule, every stream works
most actively in the upper or mountain part where its cur-
rent is swiftest. As the various headwater streams deep-
en their gullies they frequently extend them to a consider-
able distance backward ; and a very vigorous stream may
even cut its channel backward across a rid«;e or heisrht of
land. The latter then ceases to be the water-parting ; the
divide therefore " migrates " or recedes from its former

In cutting its channel backward across a ridge or height
of land a stream sometimes captures and diverts a part of
the feebler stream flowing on the opposite side of the chan-
nel (Sf' illustration, p. 113). Many of the " wind gaps" of
the Appalachian region are the results of this sort of rivt.'r
piracy. Tiiey are abandoned stream channels — aban-
doned because the former occupant of each has been capt-
ured furtlier up the valley l)y a more vigorous stream that
has crossed the height of land to get it. The Vistula
River has probal)ly obtained several of its headwater
streams by the robbery of a neighbor, and at least one


stream in Northwestern Ohio aud several in Pennsylvania
seem to have sutfered in a similar way.

Unusual Adjustments. — In selecting a new channel
or in adapting itself to the changing conditions of an old
one a river is said to adjust itself. There are several
causes which may compel a stream to change its course.
It may clog its channel, or the latter may be obstructed
by accident. Thus, by long-continued silting, the Hoang
Eiver, " China's sorrow," built its channel higher than
the divide, near the top of which it flowed. In 1852, dur-
ing a season of high floods, the river broke through its
banks. Before that time it had floAved southeasterly from


The old slreain lUninui is under the lava cap ivhich forms. Table Mountain : the present
channels are at the base of the mesa.

Kaifong into the delta of the Yangtze ; after the break its
course lay in a northeasterly direction and the river now
flows into the Gulf of Pechili.

The flood of lava that formed the ])lains of the Colum-
bia buried beneath it a long stretch of the river basin, aud
the river made a new channel around the lava flood. '^
Tuolumne Ptiver, California, was similarly buried, but
finally succeeded in making another channel through the
obstruction. It is not unlikely that Saskatchewan River
was cut in two by the rise of a height of land across its
course, the water being ponded in Lake Winnipeg and
then overflowing into Hudson Bay.

Indirectly, man is responsible for the abnormal conduct


of certain rivers, and the cause thereof is the cultivation
of the land. In order to make his land productive the
farmer must not only clear it of growing timber and de-
stroy the smaller vegetation, but he must also, in many
instances, provide a system of rapid drainage. Because
forestry, shrubbery, and sod all serve to retain water in
the soil they therefore prevent rapid drainage. The re-
moval of vegetation, on the contrary, has exactly the
opposite effect. The rainfall is rapidly collected by the
tributaries, and as quickly poured into the main stream.
As a result, high and quickly-forming floods occur.

The Ohio and the Susquehanna, especially in late years,
have suffered much from disastrous floods, and these are
mainly the result of deforesting their basins. Wooded and
grass-covered slopes are slowly drained ; denuded slopes
favor rapid accumulation of drainage waters.

Geographical Distribution of Rivers. — Rivers are
the offspring of rainfall and, as a rule, regions of great
rainfall are regions in which rivers are largest and most
numerous. This is shown in the case of the Amazon and
the Kongo. Both rivers are situated within the belt of
almost constant rains. Each has a large number of pow-
erful tributaries, and each discharges an enormous quan-
tity of water.

A river cannot develop great length and size unless the
water-shed that it drains has also a great superficial extent.
When Columbus entered the mouth of the Orinoco, he at
(^nce declared the country southward to be a continent,
for the reason that so large a river could not exist on a
small body of land.

There is no apparent law governing the distribution of
rivers except the position of slopes and the amount of
rainfall. The largest rivers are not in tlui largest conti-
nents, nor are the longest streams in regions of greatest


rainfall. The Atlantic receives the waters of more large
streams than any other ocean ; the Arctic Ocean is the
next in onlev. The reason therefor is the fact that the
largest plains slope toward the one or the other of these
two oceans.

The great ]:)lains and slopes of the Western Continent
receive the full benefit of moisture-laden winds ; and the
rivers, as a rule, reach a higher state of development than
those of the Eastern Continent. The Mississippi and the
Amazon drain each a water-shed half as large as Europe.
The Mackenzie, La Plata, Yukon, Columbia, and Colo-
rado about equal in size the great master streams of the
Old World.

The broadest part of South America is crossed by an
almost constant rain belt, and therefore is in the region
of heaviest rains. The ocean winds traverse a sweep of
about 2,500 miles before they are arrested by the Andes
Mountains ; and because precipitation covers such an
enormous area, there necessarily results a stream of vast
proportions. As a matter of fact the Amazon discharges
a greater volume of water than any other known river.

The chief plain of the Old World faces the Arctic
Ocean. It is the largest plain in the world, and is drained
by large rivers. None of them. equals the Amazon nor the
Mississippi-Missouri, however, for the reason that they
are situated in a region of very moderate rainfall.

The southern part of Europe does not extend into the
region of tropical rains ; hence the absence of large streams
on the southern slope. The southern part of Asia is un-
der the tropical rainbelt, but the drainage slope is com-
paratively short, and but few large streams have formed.
Thus it may be seen that the large plain of Eurasia is un-
favorably situated for large rivers, while, on the other
hand, the favorably situated areas are too small for the


development of great streams. The great number of
smaller rivers, moreover, compensates for the absence of
such rivers as the Amazon.

Africa possesses several large rivers, two of which, the
Kongo and the Nile, are of considerable interest. The
Kongo, like the Amazon, is an equatorial stream, and
the behavior of the two is almost identical. The Kongo
is smaller only because its basin is smaller. The Nile
is remarkable for its annual overflows, and from the fact
that, in the lower 1,200 miles of its course it receives not
a single tributar}'.

Australia possesses but few permanent streams, and
these are of small size. This continent is unfortunately
situated. It is under the Calms of Capricorn, and it con-
tains no high mountain range. The Murray-Darling is
the only river of importance. In the summer season
most of the streams disappear altogether, or else form a
succession of shallow pools.

Continental Rivers. — There are several large areas
that have no di'aiuage to the sea, and the rivers are there-
fore called con/iiienial rivers. Where is the continental
region of Eurasia ? It is drained by a multitude of riv-
ers ; name the four largest. In Africa the only large con-
tinental rivers are those flowing into Lake Chad. There

Online LibraryJacques W. (Jacques Wardlaw) RedwayElementary physical geography : an outline of physiography → online text (page 8 of 25)