Hydrosystem, Dam, and Reservoir Passage Rates of Adult Chinook Salmon and Steelhead in the Columbia and Snake Rivers

We assessed upstream migration rates of more than 12,000 radio‐tagged adult Chinook salmon Oncorhynchus tshawytscha and steelhead O. mykiss past a series of dams and reservoirs on the Columbia and Snake rivers. Most fish passed each dam in less than 2 d. Migration behavior in reservoirs and through multiple dam–reservoir reaches varied within and between years and between species. Within years, spring–summer Chinook salmon migrated more rapidly as water temperature and date of migration increased; between years, spring–summer Chinook salmon migrated fastest in low‐discharge years. Steelhead migrations slowed dramatically when summer water temperatures peaked within each year, then increased as rivers cooled in fall. Mean summer temperatures explained more between‐year variation in steelhead passage rates than did differences in discharge. Fall Chinook salmon migration rates also slowed during periods of warm water. Protracted passage times within the hydrosystem were most likely for fish from all runs that fell back over and reascended dams and for steelhead that sought thermal refugia by straying temporarily into coldwater tributaries.