157.Falls and Rapids. -In many streams the flow of water is interrupted by falls and rapids. Sometimes the course of a stream is crossed by a great break in the earth"s crust, one side of which has been raised above the other. This makes a fall, or, if the stream is able to cut down fast enough, a rapid. Falls or rapids of this kind have been pro- duced in the Colorado River.
YOSEMITE FALL.
One of the most beautiful falls in the world, due to glacial action.
Sometimes the course of a stream is changed, as was the case with many of the streams in the northern part of North America at the time of the Glacial Period. In its new course the stream may fall over a cliff, as did Niagara River which used to fall into Lake Ontario over the cliff near where Lewiston is now located. Here was developed a great fall which, owing to the kind and position of the rocks over which the river flowed, has moved back, leaving a gorge about seven miles long.
The rock layers are nearly horizontal with a hard layer at the top and softer layers below. As the water strikes the foot of the falls it drives rebounding currents against the rock wall behind it, and wearing away the softer rock undermines the harder rock at the top, which breaks off in great blocks. Thus the falls maintain an almost vertical wall behind them. These falls are about 160 feet high, one of the grandest of nature"s wonders and one of the greatest sources of water power in the world.
Falls or rapids may also be formed in a stream where it passes from harder to softer material, as from the old land to the coastal plain. The softer material is worn away faster than the hard material and the stream bed lowered more rapidly, thus forming a precipitous descent. Falls of this kind were also formed where the glacial ice forced the streams to make new channels for themselves across the upturned edges of layers varying in hardness.
The falls in the northern part of the United States were most of them formed by the rearrangement of the drainage lines at the close of the Glacial Period, and those in the southern part of the country by the more rapid wearing away of the softer rocks of the coastal plain. Thus we see that the hum of the spindle and the lathe are often but the modulated whispers of those ancient forces which thousands of years ago sorted the rock materials and built the vast continental ice palaces of the Glacial Period.
Streams which have falls and rapids have not flowed in their present channels a long time, as time is reckoned in considering the earth"s history. If they had, the falls and rapids would have been worn backand smoothed out. Thus, falls and rapids are characteristic of young rivers.
158.River Development. -The rain which falls upon a flat coun-try runs off very slowly, a large part of it soaking into the ground. Pools and lakes are formed in the inclosed basins, and sluggish streams with irregular little crooks, which show that the streams have hardly decid-NIAGARA FALLS.
A young river cutting down a layer of hard rock.
ed where they want to go, wander in the slight depressions down the gentle slopes and unite with other streams here and there until a river of ever increasing size is formed.
In some places the streams flow through lakes where they deposit their sediment, thus filling the lake basins. Here and there they pass over hard layers of rock which hold them up in falls and rapids. These they at once begin to smooth down. Rivers of this kind may well be called young, as their life work is just beginning. The Red River of the North, with its shallow narrow valley and tortuous course, and the Niagara River, with its lakes and falls, are examples of young rivers.
Where the slope of the newly exposed surface is considerable, the streams flow much more rapidly and develop their courses more quickly. The small irregularities are sooner straightened and the troughYELLOWSTONE RIVER.
A young river flowing in a deep trough.
deepened, thus forming side slopes down which run little rivulets which in time form side streams. The heads of these, like the heads of the larger streams, are constantly working back into the undissected area. Gradually the side streams develop side streams of their own, and almost the whole surface is covered with a network of streams.
As the work of erosion goes on and the streams deepen their valleys, only a few imperfectly drained remnants of the former flat surface are left here and there. These lie between the larger streams in places which the side streams have not as yet been able to reach.
Almost the entire surface is so intricately carved into drainage lines, that wherever water falls it immediately finds a downward slo pi n g sur fa c e. T h e m a i n stream by this time has probably smoothed out most of its fallsand rapids and has developed
long, smooth stretches.
A STREAM WORKING BACK INTO AN
UNDISSECTED AREA.
Here it is no longer cutting down its trough, but has only sufficient slope to enable it to bear along its load of waste. It here deposits upon its valley floor about as much as it takes away. In this part of its course a river is said to be graded. The longer a river flows undisturbed by any deformation of its valley, the fewer falls and rapids it will leave and the longer will be its graded stretches. The Missouri River near Marshall, Missouri, is an excellent example of a graded river.
Sometimes a stream becomes so overloaded with detritus, which it has acquired in a steeper part of its extent, or which has been brought to it by tributaries, that it is continually being forced to deposit some ofRIVER EROSION.
Cutting down the outer side of the curve and depositing on the inner.
its load. Thus it silts up its course and flows in a network of interlacing shallow channels. The Platte as it crosses the plains of Nebraska is an example of such an overloaded river.
THE PLATTE RIVER.
