Page:Popular Science Monthly Volume 54.djvu/56

48 it, which material is generally the product of previous erosions. Whenever the current needs all its strength to carry the material it has in suspension, together with the solid matter it is pushing along, it will have lost all its power to cause erosion. If the slope is decreased, or if the matter in suspension is increased in any manner, deposits will occur. These deposits render the slope less steep at that point but steeper below, so that the action of the water beyond will make itself felt, and by digging gradually up stream tend to restore the original slope.

In some cases there is very little erosion where the current is much stronger than the cohesive power of the soil, for the reason that the beds of the streams have been almost paved with stones that have been carried along by the propulsive action of the water.

This has in many cases produced an equilibrium between the resistance and the destroying power; in others the equilibrium has been brought about in a different way by the same natural agents. The current of a stream will very often go on causing erosion until arrested by some rocky obstacle that determines a waterfall. These falls cause breaks in the action of the water not only in stopping the erosive action in its upward march, but also in checking the velocity of the water. Then, as basins are often formed just above the falls and where the current is much less swift, matter in suspension is deposited, so that when the stream is swollen it has material to work upon, before starting to make the original slope steeper.

The subject divides itself broadly into two branches—the extinction of torrents and the correction of the water courses in valleys. In the extinction of the torrents various plans are resorted to, which give the current greater propulsive power, but at the same time they render necessary greater protection of the bed. This may be done by incasing it within walls of masonry (though other materials are used in some cases) or shortening meandering portions. In the latter plan the slope is increased, the fall being the same for a shorter distance. Currents that have been making dangerous deposits at certain points and causing dangerous erosions at others are treated by the above systems until the danger has disappeared or the money has given out. When the erosive action of the water is already too great, the material carried and then deposited by the stream is often made use of to consolidate banks that are threatened. Spurs are built out from such banks, and this tends to mend matters not only in forcing the water to take another channel, but also in causing deposits at the foot of the menaced bank.

The destructive effects of the current are arrested when the streams are not important by means of dams made of trunks of trees and wooden stakes, often strengthened roughly with stones. Where