Page:The National Geographic Magazine Vol 16 1905.djvu/341

Forecasting the Weather and Storms 305

at great intervals a stream does appear in permeable ground the flow of water may diminish as the stream progresses, the water being absorbed by the soil or sinking through it to the bed rock. Much of the water absorbed never reaches the rivers. In the Ohio Valley the amount of water drained away by the rivers is about one-fourth of the rainfall, which is the same as in Europe; in the Missouri Valley the amount is only one-eighth. These conditions have an important bearing when considering what river stages will be effected by a given rainfall and what will be the rate of rise. In an impermeable region the rivers rise rapidly and as quickly subside ; in a permeable region the rise and the fall are slower in action and the amplitude of the movement less.

In small rivers the slope may fall away at the rate of four to seven feet the mile, while in large rivers, like the Mississippi, the slope is only about one-fourth of a foot. The velocity of a river does not depend alone upon the slope, but also upon the mean hydraulic depth, the square root of the two measures determining it closely.

The regimen of a river is the history of its movements and their causes. It may be modified by a change in forest areas or in the area under cultivation. Cultivated ground allows of a much greater absorption than wild soil, and therefore holds in storage and conserves the supply for springs and streams after flood seasons have passed. It is therefore a question if civilization has not thereby considerably reduced the intensity of floods, notwithstanding the cutting away of forests, the area cleared of forests being small in comparison with the total area changed from a wild to a cultivated state ; but before a hasty conclusion is reached one should not forget to consider that forest coverings reduce to a minimum the amount of silt carried to streams, especially from steeply tilted surfaces. They hold the soil and prevent its washing away to the rivers, where it is deposited in such a way as to build up the river beds and possibly cause greater overflows than with the former larger volume of water and less silt. "Many have thought that the leveeing up of the Mississippi River will cause a building up of the bottom of the river by the confining between banks of large quantities of silt-laden water that formerly deposited most of its sediment on the adjacent flats before moving down the stream ; but here again account is not taken of the fact that the leveeing up of the river increases its depth, and therefore its velocity, and the carrying capacity of a stream increases as the cube of its velocity. It is probable that the bed of the river has not risen since a considerable portion was confined by levees. Many gauges that were established more than thirty years ago occasionally show minus readings.

The various flood scenes illustrated in this paper tell each its own story.