TECHNICAL AND SANITARY PROBLKMS 1075 



years after construction, but it has been found t at the effect of 

 this "soil stripping" is not always permanently successful and that 

 in the course of a few years heavy growths of organisms have s(jme- 

 times occurred. Where the reservoir sites are not thus cleaned 

 growths of algae are likely to be heavy during the first few \'ears 

 after construction, diminishing, however, withtime. The benefits 

 from soil stripping occur chiefly during the first few years after con- 

 struction. Whether or not there is economy in removing the soil 

 from the bottom of the reservoir depends upon local conditions. 

 Often it is advisable. In some cases it will be found cheaper not 

 to strip the reservoir bottom but to apply the money that would 

 be thus expended towards a filter plant. 



Swamp Drainage. The presence of swamps on a catchment area 

 tends to foster the growth of algae and similar organisms. If these 

 are located above a reservoir they may seed the reservoir and thus 

 increase the number of organisms likely to be found there. The 

 quality of the water may be improved in some instances by draining 

 the swamps, thus diminishing the chances of the reservoir becoming 

 seeded and decreasing the amount of organic food supply in the 

 water. When reservoirs are constructed it not infrequently happens 

 that pools are left with no outlet. Organisms may develop rapidly 

 in such pools and be washed into the reservoir after a rain. So far 

 as possible reservoirs should be self-draining. 



Elimination of Shallow Flowage. In the construction of reservoirs 

 efforts should be made to reduce the area over which the water 

 stands with a depth of less than ten feet. For in these areas of 

 shallow flowage aquatic plants are hkely to become seated and may 

 serve as a nidus for various organisms that ultimately become 

 scattered through the reservoir and give trouble. Cases occur 

 where it is wise to strip the soil from the areas of shallow flow- 

 age without attempting to strip the soil from the entire reservoir 

 bottom. 



Prevention of Pollution. Like other plants the algae in water 

 grow best when fertilized. Nitrogen, potash, phosphates, and sim- 

 ilar substances stimulate their growth. Polluted waters are. there- 

 fore, more likely to develop objectional growths of algae than the 

 same waters unpolluted. The ehmination of pollution from a 



