Fig. 171.—Miniature levees on an alluvial cone. Slope of Gray Peak, Colo. (R. T. Chamberlin.)
A permanent stream, as well as a temporary one, may develop an alluvial fan at the base of a mountain slope; but since the mountain course of the former is likely to be less steep than that of the latter, its waters suffer a correspondingly less reduction of velocity at any one point. The fan of the permanent stream is therefore likely to be relatively flat, and to stretch far down the valley. Such fans grade into valley plains. From the general principles already discussed, it is clear that well-developed fans go with relatively youthful stages of erosion, and belong normally to the upper parts of drainage lines.
Ill-defined alluvium.—There is a widespread mantle of alluvial material deposited by running water which was not organized into distinct streams. The water which runs down smooth slopes in sheets during showers carries fine earthy matter, as well as some that is coarser. These materials are largely deposited at the bases of the slopes, forming basal accumulations of greater or less extent, comparable in origin to alluvial fans. A relatively small amount of the slope wash is carried far out from the base of the declivities. It is not easy to realize the extent to which this process is taking place. There is hardly a slope without loose material, and there is hardly an acre of low land below a slope on which running water has not deposited sediment washed down from above. When it is remembered that this is as true of gentle slopes and their surroundings as of steep slopes, though perhaps not to the same extent, and that a very large part of the earth’s surface is made up of sensible slopes, or of flats at their bases, some idea of the aggregate effect may be gained.
There is another way of looking at the same question. Earthy matter is being continually transferred from land to sea, and chiefly from high land. Rarely does it start from any point distant from the shore and move uninterruptedly to it. It is transported a short distance and lodged, to be again picked up, carried forward another step in its journey, and lodged again. For a very large part of the earth’s surface it would be true to say that its mantle rock is material in transit from higher land to the sea.
Alluvial plains.—Most streams, whether heading in mountains or not, have gentler gradients in their lower courses than in their upper, and in spite of increasing volume are usually unable to carry to their debouchures all the material gathered above. The excess of load is dropped chiefly on the flood-plains of the streams and constitutes them alluvial plains.
The making of an alluvial plain usually involves both erosion and deposition. When a stream has cut its channel to grade, downward erosion ceases, or more exactly, downward cutting is, on the average, counterbalanced by deposition. So long as a stream is cutting downward rapidly, it carries away whatever débris descends the side slopes. When it approaches grade, the débris which descends the side slopes tends to accumulate at their bases, and the V-shaped cross-section of the valley becomes U-shaped (see [Fig. 172]). At about the same time the stream begins to meander, for, having lost something of its former velocity, it is more easily turned from side to side. As it begins to meander, it widens the bottom of its valley. This is the initial stage in the development of the valley flat (2 and 3, [Fig. 172]). In its meandering the stream encroaches on the talus accumulations at the bases of its valley’s slopes. The side-cutting may remove all the loose débris and even undercut the bluff as at a, [Fig. 173]. The stream’s meanders shift their positions from time to time so that the valley flat is successively widened at different points. By lateral planation, therefore, a stream tends to develop a flat as soon as it reaches grade. This is the initial part of erosion in the making of a river flat, but a flat developed by erosion alone is not an alluvial plain.
So soon as the flat developed by a stream exceeds the width of its channel, the water (except in times of flood) does not cover it all at the same time. On any part which it temporarily abandons, some débris (alluvium) is likely to be left. This deposit of alluvium constitutes the valley flat an alluvial plain ([Fig. 174]). It will be seen that the valley flat is commonly an alluvial plain from the beginning.
Fig. 172.—Diagram illustrating the transformation of a V-shaped valley into a U-shaped valley.