What will the rivers of the Pacific Northwest look like in the future? Will they be stable or unstable? Will the waters be cold and clear or warm and muddy? Will they have salmon or other species? These questions motivated our two-year study of climate warming effects on headwater streams draining the Cascade Mountains. Using a novel combination of snow, geohydrology, and sediment transport models we assessed the vulnerability of stream channels to changing peak streamflow. Our snow modeling shows that with just a 2°C warming, snowfall shifts to rainfall at all elevations, peak snowpacks occur over two months earlier, and snowpacks are reduced by over half of historical values. Our geohydrology modeling shows that greater rainfall and earlier snowmelt enhances peak winter streamflows but impacts depend on snow amount and watershed geohydrology. In spring-fed watersheds, increased winter flows are within historical bounds. In runoff-dominated watersheds, increased winter flows will exceed the historical range by up to 44% and the frequency of high flows will increase by over 100 days per year. Since streambed gravels and sand are transported during high flows, climate warming is likely to result in a dramatic increase in the amount of sediment moving through Cascade streams. Daily sediment transport rates in the surface-runoff system could potentially double. Besides affecting water quality with increased levels of suspended sediment, more frequent sediment transport events may lead to instability in the gravels where bull trout and salmon lay their eggs, making their survival less certain in the future. A key aspect of this project was engaging managers through a knowledge- to-action approach. Stakeholder dialogues were held in organized workshops and informal discussions through which we shared needs, information, and knowledge to interpret the consequences of these projected changes for water supply, threatened and endangered aquatic species, and dam operations.
