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Show also consume some of the small plankton directly), and these insect larvae and small fish, plus some plankton, comprise the principal food source for the larger fish. The basic productivity of reservoirs with bare shore lines is dependent partly upon the submerged and largely unseen shallow- wTater blanket of algae and, even more importantly, upon the invisible plankton swarms. Terrestrial insects that fall into the water furnish a supplementary source of food for fish during the summer months, but the amount of such food is correlated with the amount of land vegetation near the shore. In the Colorado River Basin most of the reservoirs have bare shore lines of varying extent so that this source of food is relatively unimportant. Water fluctuations are additionally harmful when they crowd fish into small residual pools where they suffer from oxygen depletion, lack of food and predation. Strawberry Reservoir, in Utah, . . . was built on a tributary of the Duchesne River to store water for irrigation and happened to cover a very productive meadowlike area. The production of trout in that area was phenomenal, but the oxygen demands of the abundant organic material in the basin was sufficient to suffocate the trout while under the ice of midwinter. The drawdown during the irrigation season was so great that the trout were forced to live in the oxygenless zone near the lake bottom. 10 Spawning beds, and shallows used by fish as refuges from attacks by larger fish, may be drowned out or left dry by such fluctuations. A benefit to wildlife and recreation may result from certain reservoirs if, in addition to reducing destructive floods, the release of water below the dam is so controlled as to augment a normally meager stream flow by providing additional water in late summer and fall. WATER TEMPERATURES Reservoir temperatures are a product of the complex interplay of temperatures of incoming streams and of local seasonal climatic conditions. Factors such as the relation of total volume of water to surface area exposed to the air, relative 10 Moffett, James W., letter, 1/ 15/ 46. 818271° 50- 9 depth of water, interplay of sub- surface currents, volume of incoming water, mixing and oxygenating effects of winds, and the degree and duration of seasonal climatic changes are all variables contributing to the changing subsurface " climate" of reservoirs. Complex stratification of temperature zones in the deeper lakes results from the interplay of these and other factors, and these temperature stratifications are altered more or less radically by the changing of the seasons. Naturally, this emphasizes again the need of making an individual planning study for each reservoir and the difficulty of making general statements as to the effects of reservoir construction. As previously pointed out, natural lakes and reservoirs in the higher mountains of the Colorado River Basin warm up for only a few weeks during the short summer season. Productivity is low because plant growth ceases at temperatures of around 40° F., the exact temperature varying with the species of plant. Many small organisms go into hibernation for the winter. Trout ( and other fish) feed but little at temperatures near 32° F., and become more or less dormant. Other favorable factors sometimes compensate for the short growing season of the higher altitudes. Abundant light and clear waters may favor the rapid growth of a zone of bottom- dwelling algae or aquatic mosses to a considerable depth, and the development of a large plankton population, as at Crater Lake, Oregon, where fish growth is excellent despite a water surface elevation of 6,177 feet, and summer surface temperatures of between 47° and 63.6° F. 11 Obviously, it is more important that fluctuations in water level be kept at a minimum in reservoirs at the higher altitudes because plant growth, being slower, cannot adapt itself to changing levels as rapidly as at lower altitudes. At the lower altitudes, the higher temperatures afford a potential growing season that sometimes extends throughout the year, and in desert regions particularly there may be abundant light. However, in the Colorado River Basin, a prevalent adverse factor that more than offsets favorable light and temperaure conditions is the presence of large quantities of silt, which cuts out the light, smothers bottom- dwelling food plants and spawning " Hasler, 1938, p. 94. 107 |
