Books: The Elements of Geology

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A series of surveys have determined that from 1842 to 1890 the
Horseshoe Falls retreated at the rate of 2.18 feet per year, while
the American Falls retreated at the rate of 0.64 feet in the same
period. We cannot doubt that the same agency which is now
lengthening the gorge at this rapid rate has cut it back its
entire length of seven miles.

While Niagara Falls have been cutting back a gorge seven miles
long and from two hundred to three hundred feet deep, the river
above the Falls has eroded its bed scarcely below the level of the
upland on which it flows. Like all streams which are the outlets
of lakes, the Niagara flows out of Lake Erie clear of sediment, as
from a settling basin, and carries no tools with which to abrade
its bed. We may infer from this instance how slight is the erosive
power of clear water on hard rock.

Assuming that the rate of recession of the combined volumes of the
American and Horseshoe Falls was three feet a year below Goat
Island, and ASSUMING THAT THIS RATE HAS BEEN UNIFORM IN THE PAST,
how long is it since the Niagara River fell over the edge of the
escarpment where now is the mouth of the present gorge?

The profile of the bed of the Niagara along the gorge (Fig. 39)
shows alternating deeps and shallows which cannot be accounted
for, except in a single instance, by the relative hardness of the
rocks of the river bed. The deeps do not exceed that at the foot
of the Horseshoe Falls at the present time. When the gorge was
being cut along the shallows, how did the Falls compare in
excavating power, in force, and volume with the Niagara of to-day?
How did the rate of recession at those times compare with the
present rate? Is the assumption made above that the rate of
recession has been uniform correct?

The first stretch of shallows below the Falls causes a tumultuous
rapid impossible to sound. Its depth has been estimated at thirty-
five feet. From what data could such an estimate be made?

Suggest a reason why the Horseshoe Falls are convex upstream.

At the present rate of recession which will reach the head of Goat
Island the sooner, the American or the Horseshoe Falls? What will
be the fate of the Falls left behind when the other has passed
beyond the head of the island?

The rate at which a stream erodes its bed depends in part upon the
nature of the rocks over which it flows. Will a stream deepen its
channel more rapidly on massive or on thin-bedded and close-
jointed rocks? on horizontal strata or on strata steeply inclined?

DEPOSITION

While the river carries its invisible load of dissolved rock on
without stop to the sea, its load of visible waste is subject to
many delays en route. Now and again it is laid aside, to be picked
up later and carried some distance farther on its way. One of the
most striking features of the river therefore is the waste
accumulated along its course, in bars and islands in the channel,
beneath its bed, and in flood plains along its banks. All this
alluvium, to use a general term for river deposits, with which the
valley is cumbered is really en route to the sea; it is only
temporarily laid aside to resume its journey later on. Constantly
the river is destroying and rebuilding its alluvial deposits, here
cutting and there depositing along its banks, here eroding and
there building a bar, here excavating its bed and there filling it
up, and at all times carrying the material picked up at one point
some distance on downstream before depositing it at another.

These deposits are laid down by slackening currents where the
velocity of the stream is checked, as on the inner side of curves,
and where the slope of the bed is diminished, and in the lee of
islands, bridge piers and projecting points of land. How slight is
the check required to cause a current to drop a large part of its
load may be inferred from the law of the relation of the
transporting power to the velocity. If the velocity is decreased
one half, the current can move fragments but one sixty-fourth the
size of those which it could move before, and must drop all those
of larger size.

Will a river deposit more at low water or at flood? when rising or
when falling?

STRATIFICATION. River deposits are stratified, as may be seen in
any fresh cut in banks or bars. The waste of which they are built
has been sorted and deposited in layers, one above another; some
of finer and some of coarser material. The sorting action of
running water depends on the fact that its transporting power
varies with the velocity. A current whose diminishing velocity
compels it to drop coarse gravel, for example, is still able to
move all the finer waste of its load, and separating it from the
gravel, carries it on downstream; while at a later time slower
currents may deposit on the gravel bed layers of sand, and, still
later, slack water may leave on these a layer of mud. In case of
materials lighter than water the transporting power does not
depend on the velocity, and logs of wood, for instance, are
floated on to the sea on the slowest as well as on the most rapid
currents.

CROSS BEDDING. A section of a bar exposed at low water may show
that it is formed of layers of sand, or coarser stuff, inclined
downstream as steeply often as the angle of repose of the
material. From a boat anchored over the lower end of a submerged
sand bar we may observe the way in which this structure, called
cross bedding, is produced. Sand is continually pushed over the
edge of the bar at b (Fig. 42) and comes to rest in successive
layers on the sloping surface. At the same time the bar may be
worn away at the upper end, a, and thus slowly advance down
stream. While the deposit is thus cross bedded, it constitutes as
a whole a stratum whose upper and lower surfaces are about
horizontal. In sections of river banks one may often see a
vertical succession of cross-bedded strata, each built in the way
described.

WATER WEAR. The coarser material of river deposits, such as
cobblestones, gravel, and the larger grains of sand, are WATER
WORN, or rounded, except when near their source. Rolling along the
bottom they have been worn round by impact and friction as they
rubbed against one another and the rocky bed of the stream.

Experiments have shown that angular fragments of granite lose
nearly half their weight and become well rounded after traveling
fifteen miles in rotating cylinders partly filled with water.
Marbles are cheaply made in Germany out of small limestone cubes
set revolving in a current of water between a rotating bed of
stone and a block of oak, the process requiring but about fifteen
minutes. It has been found that in the upper reaches of mountain
streams a descent of less than a mile is sufficient to round
pebbles of granite.

LAND FORMS DUE TO RIVER EROSION

RIVER VALLEYS. In their courses to the sea, rivers follow valleys
of various forms, some shallow and some deep, some narrow and some
wide. Since rivers are known to erode their beds and banks, it is
a fair presumption that, aided by the weather, they have excavated
the valleys in which they flow.

Moreover, a bird's-eye view or a map of a region shows the
significant fact that the valleys of a system unite with one
another in a branch work, as twigs meet their stems and the
branches of a tree its trunk. Each valley, from that of the
smallest rivulet to that of the master stream, is proportionate to
the size of the stream which occupies it. With a few explainable
exceptions the valleys of tributaries join that of the trunk
stream at a level; there is no sudden descent or break in the bed
at the point of juncture. These are the natural consequences which
must follow if the land has long been worked upon by streams, and
no other process has ever been suggested which is competent to
produce them. We must conclude that valley systems have been
formed by the river systems which drain them, aided by the work of
the weather; they are not gaping fissures in the earth's crust, as
early observers imagined, but are the furrows which running water
has drawn upon the land.

As valleys are made by the slow wear of streams and the action of
the weather, they pass in their development through successive
stages, each of which has its own characteristic features. We may
therefore classify rivers and valleys according to the stage which
they have reached in their life history from infancy to old age.

YOUNG RIVER VALLEYS

INFANCY. The Red River of the North. A region in northwestern
Minnesota and the adjacent portions of North Dakota and Manitoba
was so recently covered by the waters of an extinct lake, known as
Lake Agassiz, that the surface remains much as it was left when
the lake was drained away. The flat floor, spread smooth with
lake-laid silts, is still a plain, to the eye as level as the sea.
Across it the Red River of the North and its branches run in
narrow, ditch-like channels, steep-sided and shallow, not
exceeding sixty feet in depth, their gradients differing little
from the general slopes of the region. The trunk streams have but
few tributaries; the river system, like a sapling with few limbs,
is still undeveloped. Along the banks of the trunk streams short
gullies are slowly lengthening headwards, like growing twigs which
are sometime to become large branches.

The flat interstream areas are as yet but little scored by
drainage lines, and in wet weather water lingers in ponds in any
initial depressions on the plain.

CONTOURS. In order to read the topographic maps of the text-book
and the laboratory the student should know that contours are lines
drawn on maps to represent relief, all points on any given contour
being of equal height above sea level. The CONTOUR INTERVAL is the
uniform vertical distance between two adjacent contours and varies
on different maps.

To express regions of faint relief a contour interval of ten or
twenty feet is commonly selected; while in mountainous regions a
contour interval of two hundred and fifty, five hundred, or even
one thousand feet may be necessary in order that the contours may
not be too crowded for easy reading.

Whether a river begins its life on a lake plain, as in the example
just cited, or upon a coastal plain lifted from beneath the sea or
on a spread of glacial drift left by the retreat of continental
ice sheets, such as covers much of Canada and the northeastern
parts of the United States, its infantile stage presents the same
characteristic features,--a narrow and shallow valley, with
undeveloped tributaries and undrained interstream areas. Ground
water stands high, and, exuding in the undrained initial
depressions, forms marshes and lakes.

LAKES. Lakes are perhaps the most obvious of these fleeting
features of infancy. They are short-lived, for their destruction
is soon accomplished by several means. As a river system advances
toward maturity the deepening and extending valleys of the
tributaries lower the ground-water surface and invade the
undrained depressions of the region. Lakes having outlets are
drained away as their basin rims are cut down by the outflowing
streams,--a slow process where the rim is of hard rock, but a
rapid one where it is of soft material such as glacial drift.

Lakes are effaced also by the filling of their basins. Inflowing
streams and the wash of rains bring in waste. Waves abrade the
shore and strew the debris worn from it over the lake bed. Shallow
lakes are often filled with organic matter from decaying
vegetation.

Does the outflowing stream, from a lake carry sediment? How does
this fact affect its erosive power on hard rock? on loose
material?

Lake Geneva is a well-known example of a lake in process of
obliteration. The inflowing Rhone has already displaced the waters
of the lake for a length of twenty miles with the waste brought
down from the high Alps. For this distance there extends up the
Rhone Valley an alluvial plain, which has grown lakeward at the
rate of a mile and a half since Roman times, as proved by the
distance inland at which a Roman port now stands.

How rapidly a lake may be silted up under exceptionally favorable
conditions is illustrated by the fact that over the bottom of the
artificial lake, of thirty-five square miles, formed behind the
great dam across the Colorado River at Austin, Texas, sediments
thirty-nine feet deep gathered in seven years.

Lake Mendota, one of the many beautiful lakes of southern
Wisconsin, is rapidly cutting back the soft glacial drift of its
shores by means of the abrasion of its waves. While the shallow
basin is thus broadened, it is also being filled with the waste;
and the time is brought nearer when it will be so shoaled that
vegetation can complete the work of its effacement.

Along the margin of a shallow lake mosses, water lilies, grasses,
and other water-loving plants grow luxuriantly. As their decaying
remains accumulate on the bottom, the ring of marsh broadens
inwards, the lake narrows gradually to a small pond set in the
midst of a wide bog, and finally disappears. All stages in this
process of extinction may be seen among the countless lakelets
which occupy sags in the recent sheets of glacial drift in the
northern states; and more numerous than the lakes which still
remain are those already thus filled with carbonaceous matter
derived from the carbon dioxide of the atmosphere. Such fossil
lakes are marked by swamps or level meadows underlain with muck.

THE ADVANCE TO MATURITY. The infantile stage is brief. As a river
advances toward maturity the initial depressions, the lake basins
of its area, are gradually effaced. By the furrowing action of the
rain wash and the head ward lengthening, of tributaries a
branchwork of drainage channels grows until it covers the entire
area, and not an acre is left on which the fallen raindrop does
not find already cut for it an uninterrupted downward path which
leads it on by way of gully, brook, and river to the sea. The
initial surface of the land, by whatever agency it was modeled, is
now wholly destroyed; the region is all reduced to valley slopes.

THE LONGITUDINAL PROFILE OF A STREAM. This at first corresponds
with the initial surface of the region on which the stream begins
to flow, although its way may lead through basins and down steep
descents. The successive profiles to which it reduces its bed are
illustrated in Figure 51. As the gradient, or rate of descent of
its bed, is lowered, the velocity of the river is decreased until
its lessening energy is wholly consumed in carrying its load and
it can no longer erode its bed. The river is now AT GRADE, and its
capacity is just equal to its load. If now its load is increased
the stream deposits, and thus builds up, or AGGRADES, its bed. On
the other hand, if its load is diminished it has energy to spare,
and resuming its work of erosion, DEGRADES its bed. In either case
the stream continues aggrading or degrading until a new gradient
is found where the velocity is just sufficient to move the load,
and here again it reaches grade.

V-VALLEYS. Vigorous rivers well armed with waste make short work
of cutting their beds to grade, and thus erode narrow, steep-sided
gorges only wide enough at the base to accommodate the stream. The
steepness of the valley slopes depends on the relative rates at
which the bed is cut down by the stream and the sides are worn
back by the weather. In resistant rock a swift, well-laden stream
may saw out a gorge whose sides are nearly or even quite vertical,
but as a rule young valleys whose streams have not yet reached
grade are V-shaped; their sides flare at the top because here the
rocks have longest been opened up to the action of the weather.
Some of the deepest canyons may be found where a rising land mass,
either mountain range or plateau, has long maintained by its
continued uplift the rivers of the region above grade.

In the northern hemisphere the north sides of river valleys are
sometimes of more gentle slope than the south sides. Can you
suggest a reason?

THE GRAND CANYON OF THE COLORADO RIVER IN ARIZONA. The Colorado
River trenches the high plateau of northern Arizona with a
colossal canyon two hundred and eighteen miles long and more than
a mile in greatest depth. The rocks in which the canyon is cut are
for the most part flat-lying, massive beds of limestones and
sandstones, with some shales, beneath which in places harder
crystalline rocks are disclosed. Where the canyon is deepest its
walls have been profoundly dissected. Lateral ravines have widened
into immense amphitheaters, leaving between them long ridges of
mountain height, buttressed and rebuttressed with flanking spurs
and carved into majestic architectural forms. From the extremity
of one of these promontories it is two miles or more across the
gulf to the point of the one opposite, and the heads of the
amphitheaters are thirteen miles apart.

The lower portion of the canyon is much narrower (Fig. 54) and its
walls of dark crystalline rock sink steeply to the edge of the
river, a swift, powerful stream a few hundred feet wide, turbid
with reddish silt, by means of which it continually rasps its
rocky bed as it hurries on. The Colorado is still deepening its
gorge. In the Grand Canyon its gradient is seven and one half feet
to the mile, but, as in all ungraded rivers, the descent is far
from uniform. Graded reaches in soft rock alternate with steeper
declivities in hard rock, forming rapids such as, for example, a
stretch of ten miles where the fall averages twenty-one feet to
the mile. Because of these dangerous rapids the few exploring
parties who have traversed the Colorado canyon have done so at the
hazard of their lives.

The canyon has been shaped by several agencies. Its depth is due
to the river which has sawed its way far toward the base of a
lofty rising plateau. Acting alone this would have produced a
slitlike gorge little wider than the breadth of the stream. The
impressive width of the canyon and the magnificent architectural
masses which fill it are owing to two causes.: Running water has
gulched the walls and weathering has everywhere attacked and
driven them back. The horizontal harder beds stand out in long
lines of vertical cliffs, often hundreds of feet in height, at
whose feet talus slopes conceal the outcrop of the weaker strata.
As the upper cliffs have been sapped and driven back by the
weather, broad platforms are left at their bases and the sides of
the canyon descend to the river by gigantic steps. Far up and down
the canyon the eye traces these horizontal layers, like the
flutings of an elaborate molding, distinguishing each by its
contour as well as by its color and thickness.

The Grand Canyon of the Colorado is often and rightly cited as an
example of the stupendous erosion which may be accomplished by a
river. And yet the Colorado is a young stream and its work is no
more than well begun. It has not yet wholly reached grade, and the
great task of the river and its tributaries--the task of leveling
the lofty plateau to a low plain and of transporting it grain by
grain to the sea--still lies almost entirely in the future.

WATERFALLS AND RAPIDS. Before the bed of a stream is reduced to
grade it may be broken by abrupt descents which give rise to
waterfalls and rapids. Such breaks in a river's bed may belong to
the initial surface over which it began its course; still more
commonly are they developed in the rock mass through which it is
cutting its valley. Thus, wherever a stream leaves harder rocks to
flow over softer ones the latter are quickly worn below the level
of the former, and a sharp change in slope, with a waterfall or
rapid, results.

At time of flood young tributaries with steeper courses than that
of the trunk stream may bring down stones and finer waste, which
the gentler current cannot move along, and throw them as a dam
across its way. The rapids thus formed are also ephemeral, for as
the gradient of the tributaries is lowered the main stream becomes
able to handle the smaller and finer load which they discharge.

A rare class of falls is produced where the minor tributaries of a
young river are not able to keep pace with their master stream in
the erosion of their beds because of their smaller volume, and
thus join it by plunging over the side of its gorge. But as the
river approaches grade and slackens its down cutting, the
tributaries sooner or later overtake it, and effacing their falls,
unite with it on a level.

Waterfalls and rapids of all kinds are evanescent features of a
river's youth. Like lakes they are soon destroyed, and if any long
time had already elapsed since their formation they would have
been obliterated already.

LOCAL BASELEVELS. That balanced condition called grade, where a
river neither degrades its bed by erosion nor aggrades it by
deposition, is first attained along reaches of soft rocks,
ungraded outcrops of hard rocks remaining as barriers which give
rise to rapids or falls. Until these barriers are worn away they
constitute local baselevels, below which level the stream, up
valley from them, cannot cut. They are eroded to grade one after
another, beginning with the least strong, or the one nearest the
mouth of the stream. In a similar way the surface of a lake in a
river's course constitutes for all inflowing streams a local
baselevel, which disappears when the basin is filled or drained.

MATURE AND OLD RIVERS

Maturity is the stage of a river's complete development and most
effective work. The river system now has well under way its great
task of wearing down the land mass which it drains and carrying it
particle by particle to the sea. The relief of the land is now at
its greatest; for the main channels have been sunk to grade, while
the divides remain but little worn below their initial altitudes.
Ground water now stands low. The run-off washes directly to the
streams, with the least delay and loss by evaporation in ponds and
marches; the discharge of the river is therefore at its height.
The entire region is dissected by stream ways. The area of valley
slopes is now largest and sheds to the streams a heavier load of
waste than ever before. At maturity the river system is doing its
greatest amount of work both in erosion and in the carriage of
water and of waste to the sea.

LATERAL EROSION. On reaching grade a river ceases to scour its
bed, and it does not again begin to do so until some change in
load or volume enables it to find grade at a lower level. On the
other hand, a stream erodes its banks at all stages in its
history, and with graded rivers this process, called lateral
erosion, or PLANATION, is specially important. The current of a
stream follows the outer side of all curves or bends in the
channel, and on this side it excavates its bed the deepest and
continually wears and saps its banks. On the inner side deposition
takes place in the more shallow and slower-moving water. The inner
bank of bends is thus built out while the outer bank is worn away.
By swinging its curves against the valley sides a graded river
continually cuts a wider and wider floor. The V-valley of youth is
thus changed by planation to a flat-floored valley with flaring
sides which gradually become subdued by the weather to gentle
slopes. While widening their valleys streams maintain a constant
width of channel, so that a wide-floored valley does not signify
that it ever was occupied by a river of equal width.

THE GRADIENT. The gradients of graded rivers differ widely. A
large river with a light load reaches grade on a faint slope,
while a smaller stream heavily burdened with waste requires a
steep slope to give it velocity sufficient to move the load.

The Platte, a graded river of Nebraska with its headwaters in the
Rocky Mountains, is enfeebled by the semi-arid climate of the
Great Plains and surcharged with the waste brought down both by
its branches in the mountains and by those whose tracks lie over
the soft rocks of the plains. It is compelled to maintain a
gradient of eight feet to the mile in western Nebraska. The Ohio
reaches grade with a slope of less than four inches to the mile
from Cincinnati to its mouth, and the powerful Mississippi washes
along its load with a fall of but three inches per mile from Cairo
to the Gulf.

Other things being equal, which of graded streams will have the
steeper gradient, a trunk stream or its tributaries? a stream
supplied with gravel or one with silt?

Other factors remaining the same, what changes would occur if the
Platte should increase in volume? What changes would occur if the
load should be increased in amount or in coarseness?

THE OLD AGE OF RIVERS. As rivers pass their prime, as denudation
lowers the relief of the region, less waste and finer is washed
over the gentler slopes of the lowering hills. With smaller loads
to carry, the rivers now deepen their valleys and find grade with
fainter declivities nearer the level of the sea. This limit of the
level of the sea beneath which they cannot erode is known as
baselevel. [Footnote: The term "baselevel" is also used to
designate the close approximation to sea level to which streams
are able to subdue the land.] As streams grow old they approach
more and more closely to baselevel, although they are never able
to attain it. Some slight slope is needed that water may flow and
waste be transported over the land. Meanwhile the relief of the
land has ever lessened. The master streams and their main
tributaries now wander with sluggish currents over the broad
valley floors which they have planed away; while under the erosion
of their innumerable branches and the wear of the weather the
divides everywhere are lowered and subdued to more and more gentle
slopes. Mountains and high plateaus are thus reduced to rolling
hills, and at last to plains, surmounted only by such hills as may
still be unreduced to the common level, because of the harder
rocks of which they are composed or because of their distance from
the main erosion channels. Such regions of faint relief, worn down
to near base level by subaerial agencies, are known as PENEPLAINS
(almost plains). Any residual masses which rise above them are
called MONADNOCKS, from the name of a conical peak of New
Hampshire which overlooks the now uplifted peneplain of southern
New England.

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