Littoral Distribution and Drift of Selected Riffle Insects

Some factors influencing drift and benthic insects in the littoral zone of riffles were studied the summer and fall of 1971 and 1972 in the Clearwater River in north Idaho. Benthic samples were taken at three water depths (15, 30, and 45 2 2 cm) using a .093 m (1 ft ) Hess bottom sampler. Insect drift was taken on riffles at three distances from the shoreline in water depths of 30, 2 2 45 and 60 cm. Square metal frames .093 m (1 ft ) with nylon netting similar to the netting used on the Hess sampler (1.1 mm mesh diameter, 7 meshes cm) were used for drift sampling. Littoral variation in number and weight of riffle insects was shown to be affected by changes in depth and current velocity, inter acting with the date and station of sampling. This differential lit toral distribution of riffle insects resulted in variation of the lit toral community structure. In the non-fluctuating system, community diversity and diversity per individual decreased with increasing depths to 45 cm and current velocity to 1.1 m/sec. Fluctuating flows appear to reverse the order of community structure, i.e., community diversity and diversity per individual increase with increasing depth to 45 cm and current velocity to 1.1 m/sec