Impacts of climate change on key aspects of freshwater salmon habitat in Washington State


Mantua, N.J., Tohver, I., Hamlet, A.F.  2009. Chapter 6 in The Washington Climate Change Impacts Assessment: Evaluating Washington's Future in a Changing Climate, Climate Impacts Group, University of Washington, Seattle, Washington.


This study evaluates the sensitivity of Washington State's freshwater habitat of Pacific Salmon (Oncorhynchus spp.) to climate change. Our analysis focuses on summertime stream temperatures, seasonal low flows, and changes in the frequency and magnitude of peak flow events because these physical factors are likely to be key pressure points for many salmon populations in Washington State. We evaluate the sensitivity of weekly summertime water temperatures and extreme daily high and low streamflows under multimodel composites for A1B and B1 greenhouse gas emissions scenarios. Simulations predict increasing water temperatures and increasing thermal stress for salmon in both western and eastern Washington state that are slight for the 2020s but increasingly large later in the 21st century. Streamflow simulations predict that the largest hydrologic sensitivities are for watersheds that currently have so-called transient runoff streamflows, those that are strongly influenced by a mix of direct runoff from autumn rainfall and springtime snowmelt. By the 2080s, the hydrologic simulations predict a complete loss of snowmelt dominant basins in WA, and only about 10 basins remaining in the north Cascades classified as transient snow basins. Historically transient runoff watersheds will trend towards rainfall dominant basins and experience longer summer low flow periods, increased streamflow in winter and early spring, declines in the magnitude of summer low flows, and increases in winter flooding. The combined effects of warming stream temperatures and altered streamflows will very likely reduce the reproductive success for many salmon populations in Washington watersheds, but impacts will vary according to different life history-types and watershed-types. Salmon populations having a stream type life history with extended freshwater rearing periods (i.e. steelhead, coho, sockeye and stream-type Chinook) are predicted to experience large increases in hydrologic and thermal stress in summer due to diminishing streamflows and increasingly unfavorable stream temperatures. Salmon with an ocean-type life history (with relatively brief freshwater rearing periods) are predicted to experience the greatest freshwater productivity declines in transient runoff watersheds where future warming is predicted to increase the magnitude and frequency of winter flooding that reduces egg-to-fry survival rates.