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Jacques W. (Jacques Wardlaw) Redway.

Elementary physical geography : an outline of physiography online

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are many continental rivers in Australia. Practically all
of them are dry in summer and some are filled only when
an occasional cloud-burst pours a flood of water into their
channels.

The Humboldt, Carson, and Jordan are the principal
continental streams of North America. Describe their sit-
uation from an}' convenient map. What do they indicate
with reference to rainfall '? AVhat would be the probable
eff'ect on these rivers if the Sien-a Nevada ranges were no
higher than the Appalachian Mountains ? In South



126 PHYSICAL GEOGKAPHY

America the Desaguadero, the outlet of Lake Titicaca, is
the principal continental stream, although one or two of
the larger rivers in Ai'gentina are occasionally cut off
from the sea.

Economic Importance of Rivers. — Elvers are the most
important highways of commerce and, in many ways, are
the lines along which civilization and settlement penetrate
to the interior of a country. Even at the present time
merchandise can be carried by means of river navigation
for less than the cost of transporting it in any other way.
Most of the great migrations of peoples have followed the
lines of rivers, and in mountainous regions the cultivated
areas are confined mainly to river valleys. Outside the
Great Central Plain of the United States most of the rail-
ways of the country have been built along river valleys, so
that these are practically " lines of least resistance " to the
activities of a people.

QUESTIONS AND EXERCISES.— Under what conditions and at
what times is the stream with which you are best acquainted muddy ?

Note and describe any place at which the stream is cutting away its
banks.

Note and describe some place where sediment is being deposited. If
possible, account for the action in each case.

An embankment of freshly turned earth receives the full force of a
rainfall ; how will its general form most likely be affected ?

What effect has sod, shrubbery, and forestry on a surface that is ex-
posed to rain ?

Name some results that might occur if the channel of a stream were
blocked ?

How would the Mississippi be affected if the Ozark highlands were
elevated considerably higher ? (See any good topographic model or relief
map.)

What effect will the approaching old age of the Mississippi have on
the size of the Gulf of Mexico ?

On p. 1 13 is a map of Chesapeake Bay ; make a sketch-map and restore
the river channels on the supposition that the surface were uplifted
until about the lowest point is higher than sea level.



THE WASTING OF THE LAXD : RIVERS 127

Does the appearance of the Canon of the Colorado River suggest an
abundant or a scanty rainfall? How would a great increase in the
rainfall affect the scenery so far as the topography of the valley is con-
cerned ?

What does the absence of tributaries indicate concerning the rainfall
of the lower Nile ?

From the cyclopaedia, or any convenient reference-book obtain a de-
scription of the Volga and its delta.

Make a list of ten or more important cities situated on estuary
mouths; — two on or near delta mouths.

COLLATERAL READING.

Shaler. — Aspects of the Earth, pp. 143-190.

Mill.— Realm of Nature, pp. 241-251.

Davis. — Rivers of New Jersey. National Geogi-apliical Maga-
zine.

Redway. — Phy.sio^raphie Geography of the Mississippi River.
Proceedings Enginters' Club, Philudelpliia.

Mississippi River Co>I^IISSIO^^ — Map of the Alluvial Valley
of the Mississippi River.

Powell.— Physiograpliy of the United States, Monograph IL

Russell. — Rivers of North America.



NOTES.

^ On an average, about three feet of water fall each year on the
land. The rate is not uniform, however, but varies from a frac-
tion of an inch to about fifty feet. Directly and indirectly all
the Avater of the land comes from the sea and, sooner or later, re-
turns thereto.

^The term " water-shed " is often u.sed as a synonym of "di-
vide." Properly used, however, it is not a divide but a basin.

'Tlierc are a few instances in which the divide is so ill-defined
that the same pool, pond, swamp, or stream may discliarge its
waters into streams whose mouths are at a great distance one
from the other. Tliiis, Two-ocean Pond, in Yellow.stone National
Park, in liigli-w.itcr s<'ason, has two outlets — one tlirough the
Yellowstone to th(! Mi.ssissippi, the other tlirough the (Jolumljia.



128 PHYSICAL GEOGRAPHY

In other words one has Athuitic, the other Paei fie drainage. The
Cassiquiare River bifurcates, discharging siuiultaneouslj' into
the Orinoco and the Rio Negro, a tributary of the Amazon. Be-
tween the headwaters of the Pai-ana, and those of the southern
tributaries of the Amazon, the land is so flat that, in places, the
drainage is undecided.

* The cutting and the carrying power of water depends on the
speed of the current. A slight difference in the velocity makes a
very great difference in its carrying power. Water flowing at the
rate of four miles an hour will carry sixty-four times as much
material as water flowing at half that rate of speed ; that is, the
carrying power varies inversely as the sixth power of the velocity.

^ Silt is the name commonly given to matter held in suspension
in water; sediment to material that has been dropped. The
two words are often interchangeably used.

•^ Whichever process goes on at any particular locality depends
on the velocity of the current. In seasons of high water the cur-
rent may remove material, while at low-water stage it may form
a bar. That is, the middle course of a stream extends much fur-
ther down stream in high than in low water periods. In short
streams that flow in channels of considerable slope there Is prac-
tically but one course. In rivers whose waters are habitually
clear the " courses " are rarely ever well defined.

^ Davis cut-off at Palmyra bend, near Vicksburg, Mississippi, is
an example. The distance around the loop w^as twenty-two
miles ; across the neck, it was scarcely half a mile. An obstruc-
tion anchoring in mid-channel forced the current against the
narrow neck, and the latter, little by little, was cut away by the
stream. Finally the isthmus was severed and the whole flood of
the river very quickly poured through the cut. Around the loop
the fall of the river was about four inches per mile ; through the
cut, over five feet. The river scoured its channel about one hun-
dred feet in depth, and so swift was the current that more than
a week elapsed before steamboats could ascend it. The effect of
the cut-off was far-reaching, and extended both above and below
Palmyra Bend a distance of over one hundred miles.

^ Such a sti'eam is sometimes called a consequent river because its
formation is consequent upon the elevation of the plain. A river
is an antecedent stream when its existence dates before that of
some other feature. Thus Green River existed before the forma-



THE WASTING OF THE LAND : EIVERS 129

tion of Uinta Mountains, and with respect to them is an ante-
cedent river.

' At the present time the real moutli of the Hudson is near
Troy. Below this point the river is an arm of the sea, swept by-
tides throughout the whole distance. This singular condition is
due to the fact that the lower part of the river has been sub-
merged, ^inee Glacial times. The explorations of the U. S. Coast
Survey have disclosed the old channel of Hudson River from
lower New York Bay a distance of nearly eighty miles to the
southeast. Were this part of Atlantic coast again to be raised,
it is not unlikely that the river would recover its long-buried
channel.

'" At the point where the angle in the ledge is formed, the reces-
sion since 1875 has been more than two hundred feet; at the
American Fall, since 1843, it has been very slight. It is a ques-
tion of time only until the Canadian Fall shall have receded to
a line between Dufferin and Sister Islands. When this has taken
place the American Fall will have nearly or quite disappeared.
Had the conditions of a hard stratum at the top and a softer one
at the bottom been reversed, there would now be no cataract,
even had there been one at the beginning of tlie present epoch.
The softer rock would have been worn away until the perpen-
dicular front had become an incline extending to a point below
Whirlpool Rapids ; and instead of the sublime cataract, there
would now be a succession of rapids like those which mark the
passage of St. Lawrence River.

" In several other localities the Columbia has cut its channel
through similar obstructions. In at least one case the river re-
claimed its former channel by cutting through the entire thick-
ness of lava, to a depth of about 2,500 feet ; at the two "cascades "
the river is attempting to cut its channel through coulees of lava
that flowed Jicross its channel. Deschutes River, a tributary of
the Cohnnbia, is readjusting itself by cutting a new channel into
the same sheet of lava.



CHAPTER YIII

THE WASTING OF THE LAND : THE WORK OF UNDER-
GROUND WATERS

Probably almost as much water sinks into the porous
rock and the innumerable crevices of the rock-envelope as
gathers in the various external channels. The work of
telluric, or underground waters may not be quite so active
in degrading the rock envelope as are the surface streams,
but they are nevertheless important factors in the physio-
graphic processes that shape the earth's topography.
Surface streams flow quickly away in their channels, but
the underground waters must trickle slowly through chan-
nels that are ill-adapted, spending their energy not only
in forcing their way through passages that perhaps are
self-made, but also in keeping the passages clear of ob-
structions. The work of surface waters, therefore, is com-
paratively easy and simple ; that of undergroimd drainage
is vastly more difficult.

If the prevailing rock of a region be mainly of clay, or
slate, or other impervious rock, the imderground di'ainage
will be close to the surface,^ for such rocks not only pre-
vent the passage of water, but they are also insoluble. In
such cases the water must trickle through the top soil much
in the same Avay that water passes through a filter made of
sand and gravel — that is, it must flow in the spaces be-
tween the particles of rock waste.

On the other hand, if the rock of a region is mainly of
limestone, and more especially if the strata be broken and

132



UNDERGROUND WATERS



13-^



faulted,^ imdergrouud drainage is apt to be very extensive.
Not only does the water clear a passage for itself along the
lines where the rock is broken, but it also dissolves enough
of the limestone to make caverns of vast extent.

It must not be assumed, however, that these waters al-
ways remain underground. On the contrary^ they are con-
stantly in motion, and they finally emerge from their chan-
nels to reach the surface. In the study of underground
waters they may be considered of three kinds, namely —
percolating icaters, springs and artesian wells, and under-
ground streams.

Percolating Waters. — AVhen water sinks into porous
ground it fills the spaces between the grains of sand, gravel,




Water Level



DIAGRAM SHOWING THh FLOW OF PERCOLATING WATERS

or other soil. Some soils are so porous that a cubic foot
will contain more than one-quarter of its bulk of Avater.
The latter sinks through the ground until it meets a layer
of rock or clay through which it cannot pass. It therefore
accumulates until its level is as high as the rim of the im-
pervious stratum.

Flowing over the lowest part of this rim, it goes on, per-
haps to fill a similar basin lower down the slope, or possi-
bly it comes to the surface in the form of a swamp, a pond,
or a lake. If the jilain or slope is traversed by a river val-
ley a great deal of the water oozes through the soil into the
stream. In many instances waters of percolation are the
chief sup[)lies of streams.''



134 PHYSICAL GEOGRAPHY

Wells are always filled by percolatiug waters, and to ob-
tain an abundant supply it is necessary only to sink a shaft
to some point below the level of the water. Unless the
well is so shallow^ as to catch the surface drainage, the water
is usually cold and wholesome. The water of shallow wells
is apt to be impure.

If the area of porous soil is large and has a considerable
depth, an enormous quantity of water may be held. The
City of London, with its six millions of people, is supplied
with water that percolates through the adjacent chalk-beds,
and the water supplies of many of the towns and villages
of the high plains east of the Kocky Mountains are de-
rived in a similar manner.




THE WATER SUPPLY OF LOW SANDY ISLANDS
The lighter fresh water rests on the sea water.

The " sand valleys " of Western Kansas, Nebraska, and
Dakota furnish an excellent example of percolating waters."*
The storm waters falling in these valleys are almost all ab-
sorbed and held in suspension by the deep deposits of light,
pulverulent rock waste. During dry seasons the waters of
these reservoirs are about the only supply to the people
living in that region. The amount thus held in the porous
rock waste is generally sufficient to irrigate the crops that
otherwise would perish from drought.

The water supply of small and low islands is obtained in
a similar manner. The storm waters fall on the island
and immediately sink into the sand until they reach salt
water. But inasmuch as the fresh water is the lighter of
the two, it rests upon the surface of the salt water without
mixing with the latter.



UNDERGROUND WATERS



135



In many iustances the imdergTound waters are confined
between inclined strata of impervious rock. In such a
case, if the porous layer be tapped by a boring, the water
is forced up through the shaft to its normal level. Arti-
ficial springs of this character are called artesian tvells.
The " driven " or " piped " wells so common throughout
the Mississippi Yalley and the prairie region are examples
of such wells. They are shallow, however, and tap only
the superficial percolating waters. The water, moreover,
is usually brought to the surface by ordinary lifting pumps ;
it is very rare that such wells are " spouters."

In the case of wells sunk to a depth of two thousand
feet or more, the water in many instances is thought to




THE WATER SUPPLY OF ARTESIAN WELLS
The porous stratum is both covcrcii ami underlaid until impervious rock.

be forced above the surface — not by gravity, as is com-
monly supposed, but by the pressure of the air or other
gases within the reservoirs.

Along the low coast plain of Southern California several
hundred shallow artesian wells have been driven, and many
acres have been made productive. The first wells were
spouters, but at present, in nearly every instance the
water must be pumped to the surface. Many such wells
have be<'ii bored in the Sahara.^

Springs. — A small stream of Avater issuing from the
ground is called a sprimj. In some cases the water spurts
from a sloping wall, such as the face of a clift', but in gen-
eral, it gushes out of comparatively level ground near the



136 PHYSICAL GEOGRAPHY

foot of a slope. Usually the discharge does not amount to
more than a few gallons per minute, but in a few instances
it is sufficient to till the channel of a good-sized stream.*^

The storm waters that fall on porous soil sink until they
come to rock through Avliich they cannot pass, and, flowing
along the surface of this impervious layer, finally emerge
to the surface at some distance lower down. In the mean-
time, if the water has been able to make a free channel
instead of slowly percolating through the ground, it be-
comes a spring.

As a rule every spring makes its own channel. Usually
the force of the flowing water is sufficient to carry away
the lighter and finer material, thereby not only forcing a
passage, but keeping it clear afterward ; but in many cases
the water makes a channel by dissolving a part of the rock
through which it flows. If the quantity of material dis-
solved be considerable, mineral springs result. Such
springs are very common. Those at Saratoga, Vich}^, and
Carlsbad, are known all over the civilized world.

In volcanic regions, where the rocks are apt to be seamed
with fissures, the water trickles downward until it comes
in contact with heated rocks, and when it again emerges
to the surface the water may be at a boiling temperature.

So long as the mouth of a spring is lower than the sur-
face of the waters from which it is derived, the spring will
continue to flow, and will be a constant spring. If it be
situated in a region of periodical rains it is apt to be a
periodical sprmj/— flowing during the rainy season only.
If the flow depends partly on the pressure of air or other
gases, an intermittent spring may be formed.^

Geysers. — In several volcanic regions there are hot
springs, which at intervals eject copious quantities of hot
water and steam. The eruptions, unlike volcanic out-
bursts, occur with almost clock-like regularity.



UNDERGROUND WATERS



13^



The geyser differs from other hot springs in having a
long, irregular tube that extends deep into hot volcanic
rocks. The tube is formed
probably by the spring
water itself, which, Avhen
very hot, dissolves a con-
siderable amount of the
mineral silica but deposits
it on cooling.

The water that gradually
collects in the lower part
of the tube in time is heated
far beyond the temperature
at which water ordinarily
boils. For a considerable
time, tlie weight of the water
in the upper part of the
tube prevents boiling in
the lower part. Finally a
small amount of steam is
formed, and some of the
water is forced out at the
top of the spring. As soon
as this occurs, the pressure
at the lower part being re-
lieved, the water below,
that has been heated far
above the boiling point,
flashes into steam — not
gradually but instantly.

Eruptive springs of this character are not common, and
there are but three regions known in which they have been
found — Iceland, Yellowstone National Park, and Northern
New Zealand. Hot mineral springs occur in many other




A GEYSER, YELLOWSTONE NATIONAL
PARK



138 PHYSICAL GEOGEAPHY

localities, but they are not eruptive. The geyser region of
Iceland has been known for more than a centui-y. It is
situated near the group of active volcanoes and covers an
area of two or three square miles. There are about one hun-
dred eruptive springs, oue of which. Grand Geyser, spouts
a cohimn of water and steam to a height of one hundred
and forty feet. The New Zealand group is situated near
the volcano Tarawera. It is small in area, and contains
but few spouting springs.

The geyser region of the Yellowstone National Park,
Wyoming, contains several groups, mainly in the basin of
Firehole River. It comprises more than ten thousand
geysers and hot springs. Of tliis number about two score
discharge water to a height of one hundred feet or more ;
one, the Giantess, spouts a column of water two hundred
and fifty feet high, while the steam is forced nearly a thou-
sand feet higher. The eruptions occur at periods varying
from thirty minutes to about as many hours. Each is pre-
ceded b}^ a gentle overflow of water, and commonly lasts
from a few seconds to fifteen minutes, but in a few instances
continues for more than two hours. The intervals between
eruptions rarely vary more than a few minutes, but care-
ful observations show that their length is increasing, and
the energy of eruption is diminishing.

The deposition of silica from the cooling waters takes
fantastic forms. In many instances the rock thus produced
is richly colored with variegated bands. The " Pink-and-
White Terraces " of New Zealand derive their name from
this fact.

Mud Volcanoes.— Mud "Volcanoes" are hot springs
that have piled cone-shaped mounds of mud about their
vents. The mud hardens into a compact mass. Steam
and sulphurous gases are commonly the products of these
alleged volcanoes. The energy displayed is feeble, and



UNDEKGKOUND WATERS 139

the mud cones are seldom more than twenty or thirty feet
high. The mud consists of fine clay formed from the min-
eral matter of the spring. Mud volcanoes are common in
all volcanic regions.

Underground Streams. — In addition to the multitude
of surface streams, a large part of the water finds its way to
the sea — not simply by percolation but in underground
streams. Undoubtedly the run-off of most streams is
mainly above ground, but at the same time, a considerable
part of their waters flows below the surface.

There are several reasons for this. In the first place,
whenever a stream flows in a gravelly channel, a great deal
of the water must necessarily sink into the gravel and flow
along the old bed-rock bottom. The same is equally true
in the case of rivers that flow through light, sandy rock
waste, such, for instance, as those of the Basin Region,
west of the Eocky Mountains. The underground flow of
such rivers is strong even when fierce summer heat has
evaporated their surface waters.^

In many cases, too, small river channels have been ob-
literated for one purpose or auother. Noav, although the
surface flow may be destroyed the underground current is
not ; on the contrary, it is apt to be strengthened. Thus,
in some of the larger cities many small drainage courses
have been covered up in grading the streets, and in several
instances it has been found necessary to excavate these old
water-courses and sewer them.

lu New York and London the channels of many such
streams have been plotted, and drainage maps showing their
former courses are used by tlie Boards of Health in sanitary
investigation. In several instances these streams, becom-
ing obstructed, have forced their way to the surface and
flooded the streets with a deluge of water. Such experiences
are not uncommon ; they occur in ahnost every large city.'



140



PTIYSICIAL GEOGRAPHY



Of still ^a-eater interest, though not more important, are
the various " Lost " rivers. These streams receive their
name from the fact that for part of their courses they are
ordinary surface streams ; for the rest, they flow through
subterranean channels. In some instances the water of




A SINKHOLE, EDMUNSON COUNTY, KENTUCKY
The throat leading to the eavertl below has been artificially closed.



the river disappears by percolation ; in most instances the
stream pitches headlong into a " sinkhole."

In the limestone area of southern Indiana, Kentucky,
and Tennessee, underground rivers are very common.^o
One of these rivers winds its way beneath the floor of
Mammoth Cave. Its waters contain a species of fish and
two or three of insect life that have rudimentary eyes



UNDERGROUND WATERS 141

only — and indeed they have no use for perfect organs, for
never a ray of light penetrates to their abode.

Similar streams are found in Weir's Cave, in Lurav Cave,
and, in fact, in almost every limestone cavern. In Derby-
shire, England, the Hampo and the Manifold flow man}-
miles eacli through an underground passage. In both in-
stances the identity of the stream is proved by throwing a
floating body into the water above the beginning of its
underground course and capturing it w^hen it reappears.

In Southern California, where water is required for ir-
rigation, underground streams have been captured and
forced to the surface. This is accomplished b}' building a
dam across the stream at a point Avhere it emerges from
the canon to the open plain. The dam extends from the
surface of the ground down to bed-rock. The water is
thereby forced to the surface.

It is noticeable that where such submerged dams are
constructed, the artesian wells in the plain below are seri-
ously impaired — the flow of water being greatly reduced —
all of which seems to show that underground w'aters have
a much greater circulation than is generally imagined.

Physiography of Underground Waters. — Although
the work of underground waters is by no means so extensive
as those flowing above the surface, they are nevertheless of
great importance especially from an economic point of view.
Under almost any conditions water has a considerable sol-
vent power, and hot water, especially if imder pressure, will
dissolve many kinds of rock that are not aftected by cold
water ; when the solution cools, however, nuicli of this
matter is again freed from solution. In the meantime, if
the water has been forced to the surface, the substances
dissolved will l)e carried aloug and there deposited.

Sometimes the deposits are spread hap-hazard over the



Online LibraryJacques W. (Jacques Wardlaw) RedwayElementary physical geography : an outline of physiography → online text (page 9 of 25)