Artificial Lakes for Domestic Uses 21 



Water-supply Reservoirs 



Water-supply needs for towns and small cities frequently are met 

 tlirough the construction of artificial impoundments of intermediate sizes. 

 In the East, an attempt has been made to restrict trespass on these water- 

 supply reservoirs and thus to prevent their use for public recreation. Only 

 recently has this policy been reversed. In the Midwest and West, almost 

 no attempt has been made to restrict the recreational uses of water-supply 

 reservoirs, and fishing, swimming, and boating are common. Where 

 recreation is not restricted, a great deal of use may be made of water- 

 supplv reser\^oirs; however, water consumption holds priority, and where 

 or when other uses conflict with the primary use, they must be sacrificed. 

 Recently, the health departments of some states have allowed the use of 

 rotenone in water-supply reservoirs to control the overabundance of small 

 stunted fish, or dominant populations of bottom-feeding fishes responsible 

 for strirring up the bottom mud. Rotenone in dosages great enough to 

 kill fishes is nontoxic to warm-blooded vertebrates. A detailed discussion 

 of the use of rotenone will be given later. 



Reservoirs built for city and town water supplies are often created by 

 damming permanent streams or small rivers. The smallest stream capable 

 of filling the reservoir basin and also of supplying the annual needs of the 

 community is the most practical choice. This is true because the silt load 

 carried by a stream is roughly proportional to its size, and the useful life 

 of a reservoir depends upon the rate of silt deposition in its basin. 



All permanent streams contain fishes: some species can not maintain 

 their populations in impounded waters, others multiply excessively in 

 reservoirs and create a constant turbidity through bottom-rooting activ- 

 ities. These latter species spoil the fishing by reducing the visibility for 

 fish that feed by sight, and also reduce aesthetic values for swimmers 

 and boaters. 



New water-supply reservoirs stocked with bass and pan fish usually 

 go through a regular fishing cycle which requires about 6 or 7 years from 

 the time water is first impounded.-^ At the end of that time active measures 

 must be taken if recreational and aesthetic values are to be maintained. 

 These techniques will be discussed in Chapter 6. 



Most water-supply reservoirs for cities in agricultural regions are 

 relatively shallow because of the moderate slope of the land. These reser- 

 voirs are almost always thermally stratified in summer and lose their 

 supply of oxygen in the deeper water (eutrophic lakes). 



Usually in rough or mountainous regions, water-supply reservoirs are 

 comparatively deep and infertile. Because the deeper sterile waters do 

 not lose their summer oxygen supply, these lakes ( oligotrophic ) support 

 cold-water fishes such as lake trout and whitefish. 



