Historical meteorological and hydrological data were assembled to review the influence of environmental factors on patterns of adult Sockeye Salmon migration, as enumerated at Zosel Dam in the Okanogan River, WA, en route to spawning grounds in the Okanagan River, BC. We identify three locations of vulnerability for Okanagan Sockeye Salmon along the migration corridor as likely ‘control points’ exerting a disproportionate impact on upstream migration success: (1) the warmest segments of the lower Columbia River, associated with Bonneville and John Day dams; (2) Pateros Lake, above Wells dam in the mid-Columbia where salmon may hold before entering the Okanogan watershed; and (3) the 185-km Okanogan River, where summer water temperatures are typically 3-5°C warmer than Pateros Lake.

In the highly regulated flow environment of the Columbia and Okanagan watersheds, water temperature appears to be more influential than discharge on the migratory behaviour, timing, and survival of adult Sockeye Salmon. Okanogan River water temperatures have been trending upwards since the 1970s-1990s. Approximately 80% of dates during peak migration (July-August) in recent decades exceeded 20°C, up from 50% between the 1950s and 1990s. The impacts of prolonged exposure to super-optimal temperatures on adult migration success are evident from annual variations in travel time and abundance between Wells Dam and the Okanagan River spawning grounds. In cool, wet years when Okanogan water temperatures averaged <22°C during Sockeye Salmon passage (e.g. 2010 and 2011), the Wells-to-Okanagan “conversion rate” (CR) was 79-87%. In recent warm years (e.g. 2016-2018) when Okanogan temperatures were 22-23°C during adult migration, the CR fell to 43% (range 34-65%). In 2015, when Okanogan mean temperatures reached ~24°C, the CR was 8.5%. Median TT between Wells and Zosel dams averaged ~5 days (range 3-7) in 2016 and 2017, but averaged 28 days (range 13-35) due to temperature-induced migration delays during the hot spell in 2015.