COMPARISONS OF BENTHIC INVERTEBRATES BETWEEN RIFFLES AND POOLS

Benthic species assemblages in upstream and downstream ends of riffles and in pools were investigated seasonally in the first five orders of an alluvial gravel stream with distinct pool and riffle channel form. Riffles comprised < 10% of stream area and were separated by pools with extensive bedrock substrate (ca. 15-85% of total surface area) which was scoured during periodic high flow. Virtually all taxa were more abundant in riffles than in pools, except chironomids which were more equally distributed. Inconsistent results were obtained for upstream-downstream comparisons within riffles. Intermittent headwater reaches (orders 1 & 2) which supported half as many taxa retained this pattern during periods of flow, although riffles at these sites were dry from mid-June to mid-November. Pools which contained more gravel, indicating less disturbance during high flow, had a richer assemblage of benthic species than other pools. Many invertebrates in pools may have been there as a result of drift from their preferred riffle habitats, but the presence of gravel in the pools indicates less intense flow disturbance during floods, provides protection from the mild scouring that does occur during floods, and provides refugia from predators.