STREAM WORK
Most streams are incessantly at work cutting or eroding their way into
the earth's crust and carrying off the products of weathering. By this
means the general level of lands is gradually being reduced to nearer
and nearer sea level. Base level of erosion is reached when any stream
has eroded to its greatest possible depth, and a whole region is said
to be base-leveled when, by the action of streams, it has been reduced
to a practically flat condition. A region of this kind is known as a
"peneplain."
To one who has not seriously considered the matter, the power of even
moderately swift water to transport rock débris seems incredible.
A well-established law of transportation by running water is that
the transporting power of a current varies as the sixth power of its
velocity. For example, a current which is just able to move a rock
fragment of a given size will, when its velocity is merely doubled, be
able to move along a piece of similar rock sixty-four times as large!
That this must be the case may be readily proved as follows: A current
of given velocity is just able to move a block of rock, say, of one
cubic inch in the form of a cube. A cubic block sixty-four times as
large has a face of sixteen square inches. By doubling the velocity of
the current, therefore, twice as much water must strike each of the
sixteen square inches of the face of the larger block with twice the
force, thus exerting sixty-four times the power against the face of the
larger block, or enough to move it along. This surprising law accounts
for the fact that in certain floods, like the one which rushed over
Johnstown, Pennsylvania, in 1889, locomotives, massive iron bridges,
and great bowlders were swept along with great velocity. It is obvious,
then, that ordinarily swift rivers in time of flood accomplish far more
work of erosion (especially transportation) than during many days or
even some months of low water.
Few people have the slightest idea as to the enormous amount of earth
material which the rivers are carrying into the sea each year. The
burden carried by the Mississippi River has been carefully studied for
many years. Each year this river discharges about 400,000,000 tons of
material in suspension; 120,000,000 tons in solution; and 40,000,000
tons rolled along the bottom. This all represents earth material
eroded from the drainage basin of the river. It is sufficient to cover
a square mile 325 feet deep, or if placed in ordinary freight cars
it would require a train reaching around the earth several times to
contain it. Since the drainage basin of the Mississippi covers about
1,250,000 square miles, it is, therefore, evident that this drainage
area is being worn down at the average rate of about one foot in
3,840 years, and this is perhaps, a fair average for the rivers of
the earth. The Ganges River, being unusually favorably situated for
rapid erosion, wears down its drainage basin about one foot in 1,750
years. It has been estimated that nearly 800,000,000 tons of material
are annually carried into the sea by the rivers of the United States.
According to this the country, as a whole, is being cut down at the
rate of about one foot in 9,000 years. In arriving at this figure it
should, of course, be borne in mind that the average level of hundreds
of thousands of square miles of the western United States, particularly
the so-called Great Basin, is practically not being reduced at all
because none of the streams there reach the sea.
Deposition of sediment is an important natural consequence of erosion.
The destination of most streams is the sea, and where tides are
relatively slight the sediments discharged mostly accumulate relatively
near the mouths of the rivers in the form of flat, fan-shaped delta
deposits. Some rivers, like the Ganges, which carry such unusual
quantities of sediment, are able to construct deltas in spite of
considerable tides. Deltas also form in lakes. In most cases, however,
rivers enter the sea where there are considerable tides and their
loads are more widely spread over the marginal sea bottom. But in many
cases some of the sediment does not reach the mouth of the stream. It
is, instead, deposited along its course either where the velocity is
sufficiently checked, as is the case over many flood-plain areas of
rivers, or where a heavily loaded, relatively swift stream has its
general velocity notably diminished. An excellent example of the latter
type of stream is the Platte River, which is swift and loaded with
sediment in its descent from the Rocky Mountains, but, on reaching
the relatively more nearly level Nebraska country, it has its current
sufficiently checked to force it to deposit sediment and build up its
channel along many miles of its course, and this in spite of the fact
that it still maintains a considerable current. In a mountainous arid
region a more or less intermittent stream at times of flood becomes
heavily loaded with rock débris and rushes down the mountain side.
On reaching the valley floor the velocity is greatly checked and most
of the load is deposited at the base of the mountain, successive
accumulations of such materials, called alluvial cones or fans, having
not uncommonly built up to depths of hundreds, or even several thousand
feet.
[Illustration: Plate 1.--(_a_) Granite Weathering to Soil near
Northampton, Mass. Under the action of weathering all of the once hard,
fresh, mass of granite has crumbled to soil except the fairly fresh
rounded masses which are residual cores of "joint blocks." (_Photo by
the author._)]
[Illustration: Plate 1.--(_b_) Looking-Glass Rock, Utah. The rock is
stratified sandstone sculptured mainly by wind erosion, that is, by
the wind driving particles of sand against it. (_Photo by Cross, U. S.
Geological Survey._)]
[Illustration: Plate 2.--Grand Canyon of the Yellowstone River in
Yellowstone National Park. The great waterfall 308 feet high is shown.
The large swift river has here sunk its channel (by erosion) to a
maximum depth of 1,200 feet during very recent geological time, and the
process is still going on. The wonderful coloring is due to iron oxides
set free during weathering of the lava rock. (_Photo by Hillers, U. S.
Geological Survey._)]
Any newly formed land surface, like a recently drained lake bed or
part of the marginal sea bottom which has been raised into land, has
a drainage system developed upon it. In the early or youthful stage
of such a new land area lying well above sea level, under ordinary
climatic conditions a few streams only form and these tend to follow
the natural or initial slope of the land. These streams carve out
narrow, steep-sided valleys, and all of them are actively engaged
in cutting down their channels, or, in other words, none of them
have reached base level, and flood plains and meandering curves are
therefore lacking. During this youthful stage there are no sharp
drainage divides; gorges and waterfalls are not uncommonly present;
and the relief of the land in general is not rugged. A good example of
youthful topography is the region around Fargo, North Dakota, which is
part of the bed of a great recently drained lake. The Grand Canyon of
the Yellowstone River is an excellent illustration of a youthful valley
cut in a high plateau of geologically recent origin. (Plate 2.)
As time goes on, a region in youth gradually gives way to a region in
maturity, during which stage the maximum number (usually a network)
of streams in broader V-shaped valleys have developed; divisions of
drainage are sharp; the maximum ruggedness of relief has developed;
the larger streams only have cut down so near base level that winding
(meandering) courses and flood plains are well developed along them;
and waterfalls and gorges are rarely present. An almost perfect example
of a region in maturity is that around Charleston, West Virginia.
The old-age stage develops next in the history of the region, during
which only a moderate number of streams remain, most of these being
at or close to base level so that sweeping curves or meanders (Plate
4) and cut-off meanders or "ox bows" and wide flood plains are
characteristic and common. The relief is greatly subdued and the term
"rolling country" might be applied to the moderately hilly region.
Divisions of drainage are, of course, not at all sharp and the valleys
are wide and shallow. Oxbow lakes are common, but gorges and waterfalls
are absent. A region typical of old-age topography is that around
Caldwell, Kansas.
Finally, after the remaining low hills have been cut down, the region
is in the condition of a broad monotonous plain, practically devoid
of relief, over which the sluggish streams pursue very winding, more
or less shifting or indefinite courses. For the attainment of this
final stage (called a "peneplain") in the normal cycle of erosion
a proportionately very long time is necessary, because the rate of
erosion becomes slower and slower as the region is being cut down.
Then, too, some change of level between the land and the sea is very
likely to take place before the peneplain stage is reached. It is
doubtful if any extensive region was ever brought to the condition of
a perfect peneplain. Some masses of more resistant or more favorably
situated rocks are almost sure to maintain at least moderate heights
above the general plain level. Geologically recently upraised, fairly
well developed peneplains are southern New England and the great
region of eastern Canada. The remarkably even sky lines of these
regions mark the peneplain level before the uplift took place, and
occasionally masses, called "monadnocks" from Mount Monadnock in
southern New Hampshire, rise above the general level. The valleys in
such an uplifted peneplain region have been carved out by streams
since the uplift began. We have positive evidence that more or less
well-developed peneplains of considerable extent existed in various
parts of the earth at various times during the many millions of years
of known earth history.
The normal cycle of erosion which, as outlined above, tends toward the
peneplain condition may be interrupted at any stage by other processes.
An excellent case in point is the upper Mississippi Valley, which had
reached the old-age stage, even approximating a peneplain, just before
the great Ice Age. Then, during the withdrawal of the vast sheet of ice
from the region toward the close of the Ice Age, extensive deposits
(moraines, etc.) of glacial débris were left irregularly strewn over
the country, giving rise to many low hills, lake basins, and altered
drainage lines, in some cases with resultant gorge development. Some
distinct features of a youthful topography are, therefore, plastered
over what was otherwise a region well along in old age. The general
district around the Dells of Wisconsin River well illustrates this
principle.
Changes in level between land and sea which take place during the
erosion of a region may also disturb the normal cycle of erosion. For
example, a region in old age may be considerably upraised so that
the streams have their velocities notably increased. Such a region
is said to be "rejuvenated" and the streams, which are revived in
activity, begin to cut youthful valleys in the bottom of the old ones
and, after a time, the general surface of the region is subjected to
vigorous erosion and a new cycle of erosion will be carried out unless
interfered with in some way, as by relative change of level between the
land and the sea. In this connection the history of the topography in
the general vicinity of Harrisburg, Pennsylvania, may be of interest by
way of illustration of the principle just described. The long, narrow,
parallel Appalachian Mountain ridges there rise to about the same
level, causing a remarkably even sky line as viewed from one of the
summits. This even sky line marks approximately the surface of what was
a peneplain late in the Mesozoic era. Early in the succeeding Cenozoic
era, the broad peneplain was notably upraised to nearly the present
altitudes of the ridge tops. The revived Susquehanna River left the
old course which it had on the peneplain surface, and began to carve
out its present valley, while tributaries (subsequent streams) to it
developed along belts of weaker rock and thus they formed the present
parallel valleys separated by belts of more resistant rocks which stand
out as ridges. In this way, the mature stage of topography was reached.
Very recently, geologically, the region has been rejuvenated enough to
cause the larger streams to appreciably sink their channels below the
general valley floors. The reader will find a general discussion of
movements of the earth's crust in a succeeding chapter.
[Illustration: Fig. 1.--The submerged Hudson River channel is clearly
shown by the contour lines on the sea floor. Figures indicate depth of
water in fathoms. Geologically recent sinking of the land has caused
the "drowning" of the river valley. (Coast and Geodetic Survey).]
If, for example, a region along the seaboard has reached the mature
stage of erosion, and the land notably subsides relative to sea level,
the tidewater will enter the lower valleys to form estuaries and the
valleys are said to be "drowned." The large streams, or at least their
lower courses, are thus obliterated and also the general erosion of the
region is distinctly diminished. The recently sunken coast of Maine
well illustrates the idea of "drowned valleys." The drowned valley of
the lower Hudson River is another fine example.
[Illustration: Fig. 2.--Sketch maps showing how the Shenandoah River
captured the upper waters of Beaverdam Creek in Virginia. The abandoned
valley of the creek across Blue Ridge is now called a "wind gap."
(After B. Willis.)]
What is termed stream "piracy" is of special interest in connection
with stream work. By this is meant the stealing of one stream or
part of a stream by another. We shall here explain only one of the
various ways by which stream capture may be effected. One of two
fairly active streams, flowing roughly parallel to each other, is more
favorably situated and has cut its channel deeper. Its tributaries
are, therefore, more favorable to extension of headwaters and, in
time, one of its tributaries eats back far enough to tap a branch of
the less favorably situated stream so that the waters of this branch
are diverted into the more favorably situated stream. The Shenandoah
River of Virginia has been such a pirate. This river developed as a
tributary of the Potomac. By headward extension toward the south, the
Shenandoah finally tapped and diverted the upper waters of the smaller,
less favorably situated Beaverdam Creek. The notch or so-called "wind
gap" through which the upper waters of Beaverdam Creek formerly flowed
across the Blue Ridge is still plainly visible. Such abandoned water
gaps, known as "wind gaps," are common in the central Appalachian
Mountain region.
A remarkable type of river is one which has been able to maintain
its course through a barrier, even a mountain range, which has been
built across it. Thus, the Columbia River, after flowing many miles
across the great lava plateau, has maintained its course right across
the growing Cascade Range by cutting a deep canyon while the mountain
uplift has been in progress. In a similar manner the Ogden River of
Utah has kept its westward course by cutting a deep canyon into the
Wasatch Range which has geologically recently, though slowly, risen
across its path. In no other way can we possibly explain the fact that
such a river, rising on one side of a high mountain range, cuts right
across it.
A feature of minor though considerable popular interest is the
development of "potholes" by stream action. Where eddies occur, in
rather active streams, rock fragments of varying sizes may be whirled
around in such manner as to corrode or grind the bedrock, resulting
in the development of cylinder-shaped "potholes." Such holes vary in
diameter up to fifty feet or more in very exceptional cases. In the
production of large "potholes" many rock fragments are worn away and
new ones supplied to continue the work. Locally, some stream beds are
honeycombed with "potholes."
[Illustration: Fig. 3.--Grand Canyon, Arizona. (From Darton's "Story of
the Grand Canyon.")]
Strikingly narrow and deep valleys, called gorges and canyons, are
rather exceptional features of stream action. Most wonderful of all
features of this kind is the Grand Canyon of the Colorado River in
Arizona. In fact, this canyon takes high rank among the most remarkable
works of nature. The canyon is over 200 miles long, from 4,000 to 6,000
feet deep, and from 8 to 15 miles wide. Contrary to popular opinion,
this mighty canyon is not a result of some violent process, such as
volcanic action, or the sudden sinking of part of the earth's crust.
Nor is it the result of the scouring action of a great glacier. It is
simply a result of the operation of the ordinary processes of erosion
where the conditions have been exceptionally favorable. Some of the
favorable conditions have been, and are, a large volume of very swift
water (Colorado River) continually charged with an abundance of rock
fragments for the work of corrasion, and a great thickness of rock
which the river must cut through before reaching base-level. Aridity of
climate also tends to preserve the canyon form. The whole work has been
accomplished in very late geological time, and the tremendous volume of
rock which has been weathered and eroded to produce the canyon has all
been carried away by the Colorado River and accumulated in the great
delta deposit near where the river empties into the Gulf of California.
Even now the canyon is growing deeper and wider because the very active
Colorado River is still from 2,000 to 3,000 feet above sea level.
Standing on the southern rim near Grand Canyon station at an altitude
of nearly 7,000 feet, and looking down into the canyon, one beholds a
vast maze of side canyons, high, vertical rock walls which follow very
sinuous courses, giving rise to a steplike topography, and countless
rock pinnacles, towers, and mesas often of mountain-like proportions.
The side canyons are the result of erosion by tributaries to the main
river which have gradually developed and worked headward as the main
river has cut down. The mountain-like sculptured forms which rise out
of the canyon are erosion remnants, or, in other words, masses of rock
which were more favorably situated against erosion by either the main
river or any of its tributaries. All of the rocks of the broader, main
portions of the canyon are strata of Paleozoic age, arranged as a vast
pile of almost horizontal layers, including sandstone, limestone,
and shale. Some of these layers, being distinctly more resistant
than others, stand out in the canyon wall in the form of conspicuous
cliffs, in some cases hundreds of feet high. The very striking color
bands (mostly light gray, red, and greenish gray), which may be traced
in and out along the canyon sides, represent the outcropping edges
of variously colored rock layers. Far down in the canyon lies the
steep-sided, V-shaped inner gorge, or canyon which is fully 1,000 feet
deep. The rocks are there not ordinary strata, but rather metamorphic
and igneous rocks, mostly dark gray, not in layers, and about uniformly
resistant to erosion. There is reason to believe that this inner gorge
has developed mainly since a distinct renewed uplift (rejuvenation)
of the Colorado Plateau after the river began its canyon cutting. The
narrow, steep-sided inner gorge may thus be readily accounted for and
the general lack of steplike forms on its sides is due to essential
uniformity of the rock material as regards resistance to erosion.
[Illustration: Fig. 4.--Profile and structure section across the
line A-A in Fig. 3. Length of section 10 miles, vertical scale not
exaggerated. The main relief features, and the relations of the rocks
below the surface are shown. The granite and gneiss are of Archeozoic
age, and the overlying nearly horizontal strata are of Paleozoic age.
(After Darton, U. S. Geological Survey.)]
The wonderful King's River Canyon of the southern Sierras in California
is remarkable for its combined narrowness and depth. It is a steep
V-shaped canyon whose maximum depth is 6,900 feet, carved out in
mostly solid granite by the action of weathering and running water.
Some idea of the vast antiquity of the earth may be gleaned from the
fact that this tremendously deep canyon has been produced by erosion
in one of the most resistant of all known rocks in very late geologic
time! Conditions favorable for cutting this canyon have been volume and
swiftness of water and a liberal supply of grinding tools.
Among the many other great canyons of the western United States brief
mention may be made of the Grand Canyon of the Yellowstone River
in the National Park. The plateau into which the river has cut its
steep-sided, narrow, V-shaped canyon, with a maximum depth of 1,200
feet, has been geologically recently built up by outpourings of
vast sheets of lava. The large volume of very swift water, aided by
decomposition and weakening of the ordinarily very hard rock by the
action of the hot springs, has been able to carve out this deep canyon
practically within the last period of earth history. The deepening
process is still vigorously in progress. The wonderful coloring of the
rock, mostly in tones of yellow and brown, is due to the hydrated iron
oxides developed during the decay of the iron-bearing minerals of the
lava, the chemical action having been greatly aided by the action of
the hot waters. (See Plate 2.)
In regard to its origin, the marvelous Yosemite Valley, or canyon,
falls in a somewhat different category, and it is discussed beyond in
connection with the work of ice. Suffice it to say here that running
water has been a very important factor in its origin.
In New York and New England there are many gorges which have developed
by the action of running water since the Great Ice Age. Famous among
these are Ausable Chasm and Watkins Glen of New York, and the Flume in
the White Mountains of New Hampshire.
[Illustration: Fig. 5.--Sketch map showing the retreat of the crest
of Niagara Falls from 1842 to 1905, based upon actual surveys. The
retreat of the inner part of the Horseshoe Fall was more than 300 feet.
(Modified by the author after Gilbert, U. S. Geological Survey.)]
Before leaving our discussion of the work of running water, we should
briefly consider waterfalls. True waterfalls originate in a number of
ways. Most common of all is what may be termed the "Niagara type"
of waterfall. Niagara Falls merit more than passing mention not only
because of their scenic grandeur, but also because of the unusual
number of geologic principles which their origin and history so clearly
illustrate. Niagara Falls are divided into two main portions, the
Canadian, or so-called "Horseshoe Fall," and the "American Fall,"
separated by a large island. The crest of the American Fall is about
1,000 feet long and nearly straight, while the crest of the Canadian
Fall is notably curved inward upstream, and it is about 3,000 feet
long. The height of the Falls is 167 feet. Downstream from the Falls
there is a very steep-sided gorge about 200 feet deep and seven miles
long. The exposed rocks of the region are nearly horizontal layers of
limestone underlain with shales. Relatively more resistant limestone
forms the crest of the falls, and directly underneath are the much
weaker shales. Herein lies the principle of this type of waterfall
because, due to weathering and the swirling action of the water, the
weaker underlying rocks erode faster, thus causing the overlying rock
to overhang so that from time to time blocks of it already more or less
separated by cracks (joints), fall down and are mostly carried away
in the swift current. Thus the waterfall maintains itself while it
steadily retreats upstream. Careful estimates based upon observations
made between 1827 and 1905 show that the Canadian Fall retreated at
the rate of from three to five feet per year, while the American Fall
retreated during the same time at the rate of only several inches
per year. It has been well established that Niagara Falls came into
existence soon after the ice of the great Ice Age had retreated
from the district. The falls started by plunging over a limestone
escarpment, situated at what is now the mouth of the gorge seven miles
downstream from the present falls. If we consider the rate of recession
of the falls to have been always five feet per year, the length of time
required to cut the gorge would be something over 7,000 years. But
the problem is not so simple, because we know that, at the time of,
or shortly after, the beginning of the falls, the upper Great Lakes
drained farther north and not over the falls; and that this continued
for a considerable, though unknown, length of time. During this
interval the volume of water in Niagara River was notably diminished,
and hence the recession of the falls must have been slower. On the
other hand, judging by the width of the gorge, the length of the crest
of the falls has generally been considerably less than at present,
which in turn means greater concentration of water over the crest
and more rapid wear. Various factors considered, the best estimates
for the age of the falls vary from 7,000 to 50,000 years, an average
being about 25,000 years. Although this figure is not precise, it is,
nevertheless, of considerable geologic interest because it shows that
the age of Niagara Falls (and gorge) is to be reckoned in some tens of
thousands of years, rather than hundreds of thousands or millions of
years. Although waterfalls of the Niagara type are the most common of
all, it is by no means necessary that the particular rocks should be
limestone and shale.
Another common kind of waterfall may be termed the "Yosemite type,"
so named from the high falls in the Yosemite Valley of California. At
the great falls of the Yosemite, the rock is a homogeneous granite and
the undermining process does not operate. Yosemite Creek first plunges
vertically over a granite cliff for 1,430 feet to form the Upper Falls,
which must rank among the very highest of all true water falls. The
water then descends about 800 feet by cascading through a narrow gorge,
after which it makes a final vertical plunge of over 300 feet. A brief
history of the falls is about as follows. A great steep-sided V-shaped
canyon several thousand feet deep had been carved out by the action
of the Merced River which now flows through the valley. Then, during
the Ice Age, a mighty glacier plowed through the canyon, filling it
to overflowing. The granite of this district having been unusually
highly fractured by great vertical joint cracks was relatively easy
prey for ice erosion. Due to its great weight, the erosive power of
the ice was most potent toward the bottom, successive joint blocks
were removed, and the valley was thus widened and the sides steepened
or even commonly made practically vertical. (See Plate 6.) After the
melting of the ice, certain of the streams, like Yosemite Creek and
Bridal Veil Creek, were forced to enter the valley by plunging over
great perpendicular, granite cliffs which are in reality joint faces.
This type of waterfall does not retreat, but it constantly diminishes
in height by cutting into the crest. A number of other high falls of
this kind occur in the Yosemite region, and also in other mountain
valleys which formerly contained glaciers, as in the Canadian Rockies,
the Alps, and Norway, the rocks in these regions being of various kinds.
In the case of the "Yellowstone type" of waterfall a different
principle is involved, namely, a distinctly harder or more resistant
mass of rock which extends vertically across the channel of the stream.
At the Great Falls of the Yellowstone a mass of relatively fresh, hard
lava lies athwart the course of the river, while just below it the lava
has been much weakened by decomposition. The harder rock therefore
acts as a barrier, while, in the course of time, the weak rock on the
downstream side has been worn away until a waterfall 308 feet high has
developed. This waterfall does not retreat very appreciably, but it is
probably increasing in height, due both to the scouring action of the
water at the base of the fall and the unusual clearness of the river
water here, thus causing little wear at the crest. It should be noted,
in this connection, that the channel just on the upstream side of the
barrier cannot be cut down faster than the top of the barrier itself.
The famous Victoria Falls of the Zambezi River in South Africa,
represents a relatively uncommon type of waterfall. Considering height
of the fall, length of crest, and volume of water, this is perhaps
the greatest waterfall in the world. The Zambezi River, a mile wide,
plunges over 400 feet vertically into a chasm only a few hundred feet
wide and at right angles to the main course of the stream. The general
country rock is hard lava, but locally a narrow belt of the rock has
been highly fractured vertically, due to earth movements or faulting
(see explanation beyond) and therefore weakened and more subject
to weathering than the general body of the lava rock. This belt of
weakened rock has been easy prey for erosion by the Zambezi River
and the chasm has there developed. In fact the chasm is still being
increased in depth. Leaving the chasm toward one end, the river flows
through a narrow zig-zag gorge whose position has been determined by
big joint cracks. The mile-wide crest of the falls is interrupted by
a good many ledges and even small islands. The thundering noise of
this great waterfall is most impressive, but a good complete view is
impossible because most of the chasm is constantly filled with dense
spray.
Still another type of waterfall develops by the removal of joint blocks
by the action of running water. Falls of this type are fairly common
though they seldom attain really great heights. Where the rock in the
bed of a stream is traversed by well-developed vertical joint cracks,
slabs of rock cleaved by the joints may fall away due to weathering or
they may be pushed away by pressure of the water. Such a fall retreats
upstream by removal of joint blocks even in comparatively homogeneous
rocks. Taughannock Falls, 215 feet high, in southern central New York,
has developed by this manner in a shaly sandstone. The several falls
(one 50 feet high) in the famous gorge at Trenton Falls, in central New
York, have developed in this way in limestone.