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 sub- 

 side ; 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 Missis- 

 sippi, 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 de- 

 termining 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 there- 



fore a question if civilization has not 

 thereby considerably reduced the in- 

 tensity 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 in- 

 creases 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. 



