Effects of Local Topography on Streamflow and Stream Temperatures in Fish-bearing Mountainous Streams under a Warming Climate

Western Washington is home to many indigenous tribal communities that have relied on the bountiful natural resources of the region for many hundreds of years. Mountainous watersheds provide valuable fish habitat for culturally and economically important fish species, such as salmonids. Given the historically temperate weather of the Pacific Northwest that is characterized by large amounts of both rainfall and snowfall, concern has grown over the effects that a warming climate might have on freshwater fish habitat. We use the high resolution Distributed Hydrology Soil Vegetation Model (DHSVM) and coupled stream temperature River Basin Model (RBM) to examine the potential impacts of a warming climate on historically snow-melt driven watersheds in the Olympic Peninsula. An ensemble of 10 statistically downscaled GCMs from the CMIP5 group is used to force the hydrological modeling through the end of the 21st century. Here, we focus on the role of local topography (slope, aspect, elevation, etc.) in streamflow and stream temperature projections under two warming scenarios: RCP 4.5 and RCP 8.5. Mid- to high- elevation areas, where snowfall has historically been important, are most impacted by warming as snowpack becomes less abundant. Typically fast draining watersheds are likely to be more resilient to stream temperature changes, but are still likely to experience drastic streamflow changes. Increased streamflow in the winter months and decreased late spring and summer flows are projected, while stream temperatures are likely to increase most in the summer months and regularly exceed salmonid habitat temperature thresholds. The projected stream changes have the potential to greatly impact fish habitat availability and affect fish health and abundance through all of the freshwater stages of their life-cycles. Results vary by watershed, however, highlighting the importance of local topography and the need to understand individual systems when planning for the future.