After the last federal dam on the Snake River was completed in 1975, adult salmon returns dwindled, many people believed it must be the fault of the dams. However, during the same timeframe, ocean conditions dramatically shifted from cooler-than-normal temperatures to warmer-than-normal temperatures. This type of shift is a leading indicator of reduced adult salmon returns.

University of Washington – PDO Index
Kintama Research – The coast-wide collapse in marine survival of west coast Chinook and steelhead: slow-moving catastrophe or deeper failure?
NOAA Northwest Fisheries Science Center – Outlook for Adult Returns – Citizens Guide