GEOLOGIC AND HYDROLOGIC FACTORS INFLUENCING CHINOOK SALMON REDD DENSITY

An understanding of the specific factors underlying salmonid redd site selection are of critical importance to resource managers, who must often balance the demands of multiple users with the need to protect salmonid habitat. The ability to relate geologic variables to salmonid habitat could provide a valuable management tool, but a clear relationship has so far eluded researchers. We hypothesize that geologic structures and overlying, unconsolidated materials place controls on hyporehic exchange and groundwater influxes that impact the selection of spawning sites by salmonids, and that the ambiguous findings of previous investigators resulted from failure to differentiate between hyporheic exchange and groundwater discharge. In the present study, we use temperature measurements, geologic and topographic maps, and aerial photographs along a 7 km reach of Lolo Creek in Clearwater County, Idaho, to identify relationships between geology and Chinook salmon (Oncorhynchus tshawytscha) redd density. Our results indicate that geologic controls exert an important influence on redd density at the reach scale, probably because they determine the relative fractions of hyporheic exchange versus groundwater inflow and, secondarily, the slope and aspect ratio of the stream channel. Future studies will refine these preliminary conclusions, and may provide resource managers with an important tool for balancing the needs of multiple users with diverse interests.