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<br />most other locations. Production was greater than previously found for a <br />reach that was the lower limit of their occurrence in the Logan River. <br /> <br />Trout and whitefish population estimates, biomass, and production over a <br />year's time fluctuated widely in both natural and altered sites, indicating <br />the necessity to measure these parameters seasonally to adequately describe the <br />ecology of the population. <br /> <br />Mean annual densities and standing crops of invertebrates showed no <br />significant differences between sites, reflecting the heterogeneity of the <br />stream habitat and its resultant sampling variability, as well as seasonal <br />changes and the relatively short-term effects of alterations on these parameters. <br /> <br />Shannon diversity indices for invertebrates increased slightly immediately <br />following stream channel alterations, but decreased as recolonization began <br />by the more mobile families such as Chironomids, Simuliids, and Baetids. <br /> <br />All of the stream channel perturbations had the same type of effect on <br />the macroinvertebrates--lowered standing crops (numbers and weight), slightly <br />lower Shannon diversity index, and an adverse effect on production. <br /> <br />The duration of the impact by the perturbation on the benthic community <br />was largely dependent upon the rate of return of substrate stability. <br /> <br />Irrigation diversions coupled with stream channel alterations produced a <br />greater detrimental effect on fish and macro invertebrates than occurred from <br />either of these alterations alone. <br /> <br />v <br />