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Show water areas become available. With proper management, stabilized flows below dams can benefit fish, shellfish, fur animals, and other wild- life by providing a dependable sediment-free water source or by diluting pollutants. Preven- tion of floods may be beneficial to some species, but many types are naturally adjusted to periodic or annual floods and benefit from them. Irri- gated croplands and diversion canals associated with some reservoirs may benefit fur and game animals. Impoundments, on the other hand, often have the disadvantage of covering up streams and bottom lands which have been very productive of fish and wildlife. In the Western United States, valley areas which provide critical winter range for herds of bison, deer, or elk, will be inundated if the proposed Paradise, Glacier View, Wilson, and Libby Dams are constructed. In many reservoirs, water levels are subject to wide fluctuation. Such variations often cause desiccation, sedimentation, or erosion of spawn- ing beds, and the destruction of food plants along the water's edge. Fluctuations used for the control of malaria mosquitoes need not be very great, and in most cases can be fitted into a fish and wildlife plan. Toxic sprays for pest control around impoundments may be destruc- tive of fish or fish food organisms if not properly used. In general, however, the Tennessee Valley Authority and Public Health Service have found malarial prevention practices which do not conflict with, but may actually benefit, fish and wildlife. Discharges from high-altitude impoundments are often so cold that fish grow very slowly and fail to spawn. On the other hand, at low alti- tudes in desert areas it often happens that releases are cool enough to support a good trout fishery where none existed before. This has happened below Hoover Dam on the Colorado. Sometimes impoundments in valleys where out- cropping strata of finely divided material such as bentonite or shales occur, become and remain so cloudy after filling that they will not support fish or aquatic plant life. At the same time streams which are heavily burdened with sedi- ment deposit their load as they come into still water in reservoirs, and below the dam the fishing may be quite satisfactory. Reservoirs in Arid Areas.-In the arid areas of our country, water impoundments will undoubt- edly bring many benefits to wildlife. They will provide isolated populations with opportunities for fishing and hunting which they might other- wise never enjoy. Experience has shown, how- ever, that there are certain drawbacks to these arid land reservoirs. In the Rio Grande Basin, the Alamogordo Reservoir became a popular fishing area, but water shortages developed and the pool was com- pletely emptied. At Elephant Butte Dam marsh areas built up from sediment deposits at the head of the reservoir became widely known for their fish and waterfowl population, and the Bosque del Apache National Wildlife Refuge was developed there. Now the objection is raised that evapora- tion from this marsh vegetation is seriously de- pleting irrigation water supplies. Consequently, a canal is being planned wliich will drain the marsh area and thus greatly reduce the value of the area for wildlife. An identical situation has developed at the head of Havasu Lake behind Parker Dam on the Colorado, although here the main objection is that sedimentation and marsh development are causing flood damage. Canalization has already spoiled much of the fine marshland in the wildlife refuge established there. In another case, the Platboro Reservoir on the Rio Grande will inundate an excellent fishing stream, the Conejos River. It will also com- pletely dry the river below the dam. The fishing opportunities which the reservoir may create cannot be depended on to replace those destroyed, for downriver interests can demand complete emptying of the reservoir during dry periods. In spite of many conflicts, however, impound- ments will contribute to the general availability of hunting and fishing throughout the country, especially when they are properly managed and supplemented with wildlife refuges, diked-ofF areas, subimpoundments, and watershed man- agement and pollution control. Tennessee Valley Authority.-In the South- east, TVA has provided the people with a great 263 |