BENTHIC INVERTEBRATES AND SEDIMENT CHARACTERISTICS IN MAIN CHANNEL HABITATS IN THE LOWER COLUMBIA RIVER

Benthic invertebrate communities in the Columbia River between the low ermost dam and the upper estuary have been little studied. We describe the abundance of benthic invertebrates and sediment characteristics i n main channel habitats in the lower Columbia River and examine the re lationships between densities of specific benthic invertebrates and se diment characteristics and water depth. Using a 0.1-m(2) Van Veen grab sampler, we collected benthic samples in seven areas of the river fro m River Kilometer(RKm) 121 to 211 in April and September 1988 and 1989 . Common taxa collected, included Turbellaria, Oligochaeta, the bivalv e Corbicula fluminea, the amphipod Corophium salmonis, Chironomidae la rvae, and Ceratopogonidae larvae. Overall, Corbicula fluminea and Cera topogonidae larvae were consistently the most abundant benthic inverte brates. The relationship of benthic invertebrate densities between and within years, months, and areas varied substantially between the six predominant taxa. The effect of water depth and sediment characteristi cs on benthic invertebrate densities, after accounting for the influen ce of year, month, and area, varied between the six predominant taxa i n this study. Water depth was significant (P less than or equal to 0.0 5) for densities of three of the six taxa, sediment percent silt/clay significant for four, and sediment grain size and percent volatile sol ids significant for one each. Results from our study are probably appl icable to a large area of main channel habitats in the Columbia River downstream from the lowermost dam. Because benthic invertebrates, part icularly Corophium salmonis, are primary prey for many fishes, the sus tained health of benthic invertebrate populations in the lower Columbi a River is of concern.