402 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 39 



red-colored water. Scraping and cleaning the pipes in order to 

 regulate the operation of the reservoir so that only water containing 

 low amounts of carbon dioxide enters the system somewhat reduces 

 the difficulty. In the late summer months, the addition of lime to 

 the water also prevents trouble. 



SOURCES OF WATER SUPPLIES 



The main reliance for the water supply is on rivers of various kinds, 

 large and small, rapid and sluggish, pure and polluted. They also 

 vary in the number and kinds of organisms to be found in them. 

 The free-floating plankton population drifts with the current, feeding 

 and reproducing as best it can. There is no stability or permanence 

 for it even when the river is low and flowing fairly slowly but stead- 

 ily, and floods are disastrous to the plankton. The river is fed from 

 ponds and streams with quiet backwaters, and from all of these, 

 plants of different kinds are passing into the main stream and re- 

 plenishing the supply. Most rivers receive land washings, street 

 drainage, and sewage drainage that carries the intestinal bacteria 

 usually found in human and animal excrement. All of these intestinal 

 bacteria soon lose their vitality in the water and sink to the bottom, 

 thus decreasing in number between towns. The construction of 

 large quiescent basins in which sedimentation can occur assists in the 

 natural recovery from pollution, or the so-called self-purification of 

 rivers. The water of such rivers is, hov/ever, unfit for domestic use 

 until it has been chemically treated. 



The towns in hilly districts are able to obtain their water from 

 natural or artificial lakes. Usually, the land immediately surround- 

 ing and emptying into lakes and reservoirs is cleared of dwellings. 

 Access to the lakes and reservoirs is restricted so that the water may 

 be kept uncontaminated. In the temperate zone all lakes of con- 

 siderable depth overturn twice each year, usually in the spring and 

 in the fall. This phenomenon is caused in the autumn by the chilling 

 of the surface waters, which makes them heavier than the lower 

 waters. In the spring the slight warming of the surface waters, 

 usually from 35° F. to 39° F., the temperature of the greatest density 

 of water, causes a similar overturn. Normal stratification of the 

 organisms in the lake is restored in 3 or 4 weeks' time in each case. 

 This vertical movement causes a temporary commingling of the plant 

 and animal organisms at the surface with those at the bottom. 



STORAGE OF WATER 



The advantage of storing water was demonstrated by Percy Frank- 

 land, who pointed out that the leap over Niagara Falls left the bac- 

 teria unharmed, whereas they disappeared very quickly in the quiet 



