The Effects of Reduced Stream Discharge on Insect Drift and Stranding of near Shore Insects

The effects of seasonally reduced stream discharge on insect drift and stranding were studied in two experimental channels on the Grande Ronde River, Oregon. Five experiments were conducted (spring, summer and fall, 1980 and spring and fall, 1981) at three test flows (0.57, 0.28 and 0.03 m3/sec). Insect samples were taken with a modified Hess sampler and standard drift nets. Reduced stream discharge caused catastrophic drift in the test channel with drift peaking at night. Simulium sp. and Baetis tricaudatus were the principal drift components. Evidence of stranded insects in the dewatered zone was greatest in the fall, least in the spring.