Water Fluctuation, A Detrimental Influence On Trout Streams

Stream fluctuations strongly influenced the biotic populations of three Colorado trout streams during a three year water quality study conducted on the streams. Extreme water fluctuations (94% variation in surface area), combined with stream bedload accumulations, reduced a productive trout water to a non-productive series of intermittent pools during the course of the study. One study station produced the highest consistent production of benthos and the largest standing crops of trout in numbers. According to weight, however, the same station produced the fourth largest standing crops of trout. The discrepancy was attributed primarily to adverse feeding conditions for trout; a result of stream flow reductions during summer months. Rapid reductions in stream flow produced an abnormal concentration of benthos at another station, followed by a rapid decline in the benthos population within a two week period. Unusual oxygen deficits were recorded in one instance, with oxygen levels lowest during photosynthetic periods and highest during periods of darkness. Oxygen concentration deficits were traced to two factors, decomposing cow manure in the stream and low stream flow. Egg survival studies pointed out other adverse effects of stream fluctuation. Artificial redds and baskets containing rainbow trout eggs were placed in stream gravels of 2 of the 3 study streams. High stream flows at one station, due to a sudden release of water from an upstream reservoir, disinterred 75% of the buried eggs. Viability of the remaining eggs appeared to be lowered. Sharply declining water levels at a lower station on the same stream left all of the artificial trout redds and baskets free of water 1 or 2 days prior to fry emergence; after the eggs had withstood flood conditions. Cessation of water releases from the same reservoir that produced flood conditions caused the decline in water levels. At a third station located on a different stream, eggs were also exposed to air when the stream level receded. This time, water recession was due to natural rather than man caused conditions. The only vascular plant common to all three drainages was Veronica americana. Veronica was found on moist ground, sand bars and emersed, seemingly well adapted for areas of extreme water fluctuation. Other macroscopic aquatic plants were common only in George Creek, the stream with the most stabilized stream flow.

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