Defining Flow Windows for Upstream Passage of Adult Anadromous Salmonids at Cascades and Falls

Abstract Variations in streamflow at falls and cascades can create transitory barriers to upstream passage of adult anadromous salmonids. In this study, we evaluated the ability of six anadromous salmonids (coho salmon Oncorhynchus kisutch, pink salmon O. gorbuscha, chum salmon O. keta, sockeye salmon O. nerka, Chinook salmon O. tshawytscha, and steelhead O. mykiss) to pass five sets of falls or cascades within Ward Creek, Alaska, a stream regulated by Connell Dam. The study focused on determining suitable instream flow releases to afford passage. Each set of falls was surveyed under three flow conditions (about 0.91, 1.34, and 3.0 m3/s), and access portals were identified. Using computed stage—discharge relationships and published swimming and jumping criteria, we evaluated the falls barrier potential over a range (0.23–11.33 m3/s) of flows. Analysis indicated pink salmon and chum salmon would be relegated to the area below the lower two falls. Flows affording passage over all five falls for the other species (excluding Chinook salmon) were defined as “flow windows” and ranged from 0.66 to 3.01 m3/s. Flow windows varied by species; the narrowest range of acceptable passage flows was found for sockeye salmon, the widest was found for steelhead, and the range for coho salmon was intermediate. Escapement surveys generally confirmed our analysis; pink salmon and chum salmon were only found below the lowermost falls. The flow windows for passage were compatible with a set of instream flow recommendations derived via a PHABSIM‐based study but were incompatible with the highest flow recommendation based on the method of Tennant (1976). Care must be taken when evaluating instream flow needs to ensure that all flow‐sensitive factors are considered.