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Show mon and widespread reservoir fishing, it is likely that the attraction of these regions will be considerably less. Converting natural lakes into reservoirs.- In the Colorado River Basin, the enlargement and conversion of a natural lake into a reservoir rarely, if ever, will increase biological productivity and benefit fishing or related recreation. Raising the level of the lake obviously is for the purpose of using the additional water, which means that a considerable fluctuation will take the place of the former stability. The additional volume of water will not bring a corresponding increase in productivity. Only the addition of permanent, nonfluctuating, shallow water can do this. The prospect of such a loss of original productivity and recreational value, without any corresponding recreational gain, is illustrated by a plan, subsequently abandoned, to dam beautiful San Cristobal Lake on the upper reaches of Lake Fork of the Gunnison River. Reservoirs in arid regions.- More often in the Colorado River Basin the creation of a lake will introduce recreational activities and be of distinct value to the district. Many of the reservoir sites are in arid regions where water is a welcome sight and, though some of the reservoirs may not be ideal for recreational use, they will still have some recreational value. Frequently the water area will provide an opportunity for a number of new enterprises such as cabin camps, boat equipment, supply and storage, fishing supplies, and refreshment establishments. The construction of a large reservoir on a desert stream profoundly alters the immediate environment, and often improves it greatly for fish and some other kinds of wildlife. This is particularly true if the stream above the lake is heavily silt- laden. Water entering the lake loses its velocity sufficiently to deposit this silt. Productivity of the resulting clear water in a warm environment of high light intensity may be increased a thousandfold, depending upon the degree of water- level fluctuation. The improvement in the environment resulting from reservoir construction on a desert stream is not limited to the lake itself, and may be even greater in a section of the stream below the lake. The Colorado River, for a 50- mile stretch below Lake Mead, affords an excellent example of this kind of improvement. Water discharged from the lower levels of this deep reservoir ranges approximately between 54° and 61 " T., 1 whereas temperatures for the undammed Colorado River at Yuma range between 40° and 90° F., with the normal variations perhaps 10° F. less extensive than this. 2 Violent churning of the water as it passes into the open air from the turbine outlet of the dam renews the high oxygen content. The silt- free water flows for 50 miles or more over clean gravel shoals that are covered by an almost continuous blanket of green algae. 3 Growth of such bottom plants in the intense light is rapid, and is uninterrupted because the absence of a severe winter climate permits water temperatures to remain fairly constant throughout the year. The result is that fish- food organisms also multiply enormously and continually. Trout, which normally could not exist within scores or hundreds of miles of this stretch of river, have been introduced with great success. They grow rapidly and so uniformly that the age determinations on the basis of growth rings on scales have been futile. 4 Reservoir construction on desert streams can be of great benefit to recreation when, as in the case of Lake Mead, it substitutes a large, clear lake suitable for warm- water game fish, together with a section of Transition Zone trout stream, for the previous turbulent, silt- laden, and unproductive river waters. By contrast, Lake Havasu, a municipal water- supply reservoir 112 miles below Lake Mead, is far less productive of fish and other aquatic life because of the difference in its construction and functions. Although the reservoir is 42 miles long, the dam raises the water level only 75 feet at maximum, and the discharge occurs at only 50 feet below this extreme high- water line. For this reason, the stream below the dam, though silt free, is but little cooler in summer than the undammed river and therefore is unsuitable for trout, though favorable for warm- water fish. 5 1 Moffett, loc. cit., p. 77. 2 Dill, op. cit., p. 130. 3 Moffett, loc. cit., p. 79. 4 Moffett, loc. cit., p. 83. 5 Dill, op. cit., p. 112. 103 |
