io8 THK IMPROVEMENT OF RIVERS. 



"For such industrial purposes as the operation of factories and the like many 

 reservoirs have been constructed bo^h in France and in this country. They arc gen- 

 erally of small capacity, and costly per unit of water stored, but profitable i>n account 

 of the great use made of the water. Some of these reservoirs serve an important pur- 

 pose in protecting the valleys below from flixnls. 



Effects on Floods." E very reservoir built along the course of a stream is, to 

 some degree, a protection against firxxls in the valley below. The extent of this pro- 

 tection depends, of course, almost entirely on the ratio of its capacity to the flood dis- 

 charge. A reservoir that can store the entire flow of a stream is an absolute protection 

 against floods for a considerable distance below. It is difficult to propose any general 

 rule for the extent of this control, but, assuming a general similarity of watershed, it 

 would seem not unreasonable to say that it ought to be decisive to at least such a dis- 

 tance below as will give an additional watershed to a stream equal to twice that above 

 the reservoir. This is simply saying that, in the general case, the reduction of a flood 

 wave by one-third of its volume will rob it of its destructive character. 



" But in a great many cases this control extends very much farther. For example, 

 in the case of a flood caused by the rapid melting of snows in the mountains, reservoirs 

 below which can impound this flood will protect the entire valley so far as its destructive 

 influence would otherwise have reached. When it is remembered that the volume 

 of a destructive flood is only a part probably always less than half of the total flow' 

 of a year, it will be admitted that a storage capacity equal to one-fourth of the run-off, 

 well distributed throughout a watershed, will practically eliminate the evil effects of 

 floods in its streams, and supply a percentage sufficient for the purposes of irrigation. 



"It is not necessary, though important, that a reservoir should be empty when a 

 flood comes. Even if full, it still moderates the flow of the stream below, the effect 

 varying directly with the superficial area of the reservoir when full, and inversely with 

 the capacity of the spillway. In this respect it acts precisely as does a natural lake. 

 For example, if the spillway of a reservoir or the outlet of a natural lake be of such 

 dimensions as to require a considerable increase in the depth of water to give much of 

 an increase of discharge, every increment of this depth of outlet means also an incre- 

 ment of the same depth over the entire reservoir. A flood passing such a reservoir 

 will be reduced by the storage resulting from this increment, and before it can produce 

 a full discharge it must fill the reservoir to the necessary height above the bottom of 

 the spillway. A large reservoir is, therefore, even when full, always a perfect protection 

 against sudden floods. In the case of long-continued floods it greatly retards the arrival 

 of maximum effect and gives ample notice of its approach. 



" In fact, this is a very important feature of reservoir action, even where the capacity 

 of the reservoir is not sufficient entirely to prevent the flood. It does prevent freshets 

 that is, sudden floods and in smaller streams it is often the suddenness quite as much 

 as the magnitude of floods that causes damage and loss of life. 



" A reservoir ceases to be any protection if a flood continues long enough to fill it 



