University of California Water Resources Center

Freshwater mussels in California's rivers are potentially very useful as indicators of watershed health and recorders of watershed changes. We report the occurrence and habitat of mussel populations within a continuous 8-km section of the South Fork Eel River in the Northern Coast Range of California. The primary goals of our study were 1) to compile information on species composition and population density, and 2) to examine whether spatial distribution and variability were related to geomorphology and hydrology. We found numerous individuals of 2 species (Margaritifera falcata and Anodonta californiensis), with the spatial distribution of both species characterized by high variability. Mussels in this system live almost exclusively in pools (with a few in runs), near the channel banks, and especially among sedge root-mat substrate. High discharges almost certainly provide more of a constraint on the distribution and persistence of mussels in the South Fork Eel than do low summer flows, so we used the Hydrologic Engineering Center's River Analysis System (HEC-RAS) hydraulic model to estimate physical conditions during high flows when in-channel investigations were not feasible. In all flow regimes (summer, winter, 5-y flood, and the largest floods on record), mussels were found in areas of lower boundary shear stresses and lower velocities. Our study suggests that, at various spatial scales, mussels appear to be distributed in a manner that protects them from the highest flow-induced stresses.