WATER 109 



for the boats. And in addition to that, the deposits of silt in the 

 river-bed fill the channels with dangerous bars and shallows. 



The fresh water of rivers and streams is the home of fish, 

 which are a source of valuable food. But fish do not thrive in 

 silt 'laden flood waters. When the water is low it is even more 

 difficult for them to survive. In normal conditions, an even" 

 flowing stream will carry off a certain amount of natural silt, 

 sewage, and industrial waste. Poisonous parts of this waste 

 will be so diluted that they may be harmless. But when there 

 is little water, these poisonous substances are so concentrated 

 that the fish are killed. 



Farmers who irrigate their lands with river water are most 

 dependent on an even flow. , Floods wash out their irrigation 

 ditches and cover their fields with infertile silt and gravel. Low 

 water during the hot dry growing season may be disastrous. 



These are direct users of river water. The indirect users of 

 this water are also harmed by the uneven flow of streams. 

 These indirect users are concentrated primarily at the point at 

 which fresh water meets the salt water of the ocean. The more 

 fresh water that flows from a river into the sea, the more it 

 pushes the salt water away from its mouth. This creates a 

 sort of twilight sone in which there is a delicate balance be' 

 tween fresh and salt water. Shrimps, oysters, and lobsters, all 

 valuable food, can exist only in the twilight zone. When the 

 flow of streams is uneven, this balance is disturbed and, the 

 marine animals which depend on it are destroyed. 



CONTROL OF WATER FLOODS 



None of these problems of water is new. Consequently, as 

 they have become increasingly serious, men have tried to work 

 out some system of control of water. It is quite natural that as 

 the difficulty became pressing in one community, the people of 

 the community tried to solve it in their own way. Thus most 

 of the efforts to control water are attempts to escape the effects 



