Temporary Staging of Columbia River Summer Steelhead in Coolwater Areas and Its Effect on Migration Rates

We used radiotelemetry to evaluate the temporary staging of adult migrating steelhead Oncorhynchus mykiss into nonnatal tributary rivers of the Columbia River and to determine the effects of staging behavior on migration rate. By monitoring the movement patterns of 2,900 individual steelhead over 3 years (1996, 1997, and 2000), we determined that an average of 61% of the steelhead destined for upstream areas temporarily staged in one or more tributaries in the lower Columbia River for durations from less than 1 h to 237 d. Median residence time varied significantly by tributary used and year and, based on canonical correlation analysis, was correlated with main‐stem Columbia River water temperature. Steelhead that temporarily staged in tributary rivers migrated through the lower Columbia River significantly more slowly than steelhead that did not use tributaries. Use of coolwater tributaries as thermal refugia during warm summertime conditions significantly influences the migratory behavior of Columbia River adult steelhead. Our results highlight the need to preserve the water quality parameters of existing cooler‐water Columbia River tributaries and to rehabilitate watersheds that historically maintained cooler‐water tributaries as sources of thermal refugia for adult summer steelhead returning to the basin.