Hyporheic discharge of river water into fall chinook salmon (Oncorhynchus tshawytscha) spawning areas in the Hanford Reach, Columbia River

Fall chinook salmon (Oncorhynchus tshawytscha) spawned predominantly in areas of the Hanford Reach of the Columbia River where hyporheic water discharged into the river channel. This upwelling water had a dissolved solids content (i.e., specific conductance) indicative of river water and was presumed to have entered highly permeable riverbed substrate at locations upstream of the spawning areas. Hyporheic discharge zones composed of undiluted ground water or areas with little or no upwelling were not used by spawning salmon. Rates of upwelling into spawning areas averaged 1200 L.m(-2).day(-1) (95% CI = 784-1665 L.m(-2).day(-1)) as compared with approximately 500 L.m(-2).day(-1) (95% CI = 303-1159 L.m(-2).day(-1)) in nonspawning areas. Dissolved oxygen content of the hyporheic discharge near salmon spawning areas was about 9 mg.L-1 (+/- 0.4 mg.L-1) whereas in nonspawning areas, dissolved oxygen values were 7 mg.L-1 (+/- 0.9 mg.L-1) or lower. In both cases, dissolved oxygen of the river water was higher (11.3 +/- 0.3 mg.L-1). Physical and chemical gradients between the hyporheic zone and the river may provide cues for adult salmon to locate suitable spawning areas. This information will help fisheries managers to describe the suitability of salmon spawning habitat in large rivers.