Reservoir manipulations and benthic macroinvertebrates in a Prairie River

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Samples were taken on the Tongue River, Montana, USA, during 1974 and 1975 to determine the distributions and abundances of the benthic fauna after various reservoir manipulations. The upper cold water section, influenced by hypolimnial discharge from the Tongue River Reservoir, was impoverished in insect fauna and dominated by the molluscs Physa and Sphaerium. The lower warm water sections of the river contained two communities determined primarily by turbidity and periphyton cover. The upper warm water area was dominated by Strophopteryx and hydropsychid caddis larvae. The lower river was dominated by Cheumatopsyche. The summer fauna, in the warm water area, was dominated by short-lived mayfly species. During the summer, 1975, the cold water section was invaded by many insects due to warming of the area when no hypolimnion was formed in the reservoir. Invasion was apparently due to increased thermal fluctuations which caused diapause eggs to hatch and influenced the upstream migration of older nymphs and larvae. Results of drift and distributional samples after closure of the Tongue River Reservoir Dam for repairs showed that massive drift of all invertebrates began at a discharge of 130 c.f.s. (3.68 m3/sec), a drop from 190 c.f.s. (3.38 m3/sec) over a period of three days. Community composition was radically altered by reduced discharge.