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Show grounds, and suffocates fish. If the reservoir is large enough to provide clear water beyond the area of silt deposition, production of plant and animal life may be extremely vigorous, provided water- level fluctuations are not extreme. The most favorable growing temperatures for trout under natural conditions appear to be between 50° and 70° F., 12 with 70° F. close to the upper safe limit, and 80° F. too high if sustained for any length of time. 13 Trout require water of a higher oxygen content, but can endure lower winter temperatures than the warm- water fish, such as bass. The latter require warmer waters for feeding and spawning. 14 The general life zone requirements of the two types of fish already have been discussed in Chapter I. FERTILITY OF THE RESERVOIR Aquatic plants, like those on dry land, require fertile soil containing organic matter, as well as warmth, for maximum productivity. For this reason, many reservoirs in the Colorado River Basin will be unable to compare in lush productivity with reservoirs in other parts of the country that have a richer soil. 15 The silt, resulting from both natural and man- caused erosion, that collects in so many of the lower reservoirs of the basin, is composed largely of inert subsoil and lacks the required i2 Needham, 1938, p. 313. is Wales, 1939, p. 303. 14 Dill, 1944, p. 130. 15 Moffett, 1942, p. 80. organic matter. Reservoirs in the higher mountains, though more fertile, are handicapped in their productivity by prevailing low temperatures. Thus, in fertility as in other characteristics, it will be seen that few reservoirs in the basin will combine all of the elements necessary for maximum productivity. Aquatic biologists have almost universally noted that new reservoirs exhibit a relatively high rate of productivity during the first few years following construction, but that this initial productivity eventually drops to a lower and more permanent level. Lake Mead is a good example. This above- normal production, I believe, can be attributed to the accumulation of organic materials on the floor of the reservoir site which are suddenly brought into the aquatic complex by the new waters as they flood them for the first time. I have noticed this phenomenon in several reservoirs of the Colorado River drainage in Utah and also in waters of California ... I am ( also) of the impression that as soon as the first ' bloom' of production in most high mountain reservoirs is over, the resulting production is not much greater than what might have been expected from the stream before the reservoir was constructed. This assumption is likely to hold in steep- sided, mountainous reservoirs with a minimum of shoal water. Naturally this comparison cannot be made in all instances. 16 16 Moffett, James W., lit. cit. 108 |