MODELING THE EFFECTS OF CLIMATE CHANGE ON HYDROLOGY AND STREAM TEMPERATURE IN THE NORTH FORK OF THE STILLAGUAMISH RIVER BASIN

The North Fork of the Stillaguamish River in northwest Washington State is a valuable regional water resource and critical habitat for endangered salmon species. The basin is about 730 km² with relief ranging from 55 m to 2100 m. About 19% of the basin is above 1000 m elevation and is snow dominated in the winter months. Snowpack is highly sensitive to fluctuations in temperature during mild maritime winters and is the main contributor to spring and summer streamflow in the Stillaguamish River. Climate change in the Pacific Northwest is projected to increase winter temperatures, leading to an increase in precipitation falling as rain. To assess shifts in snowpack, streamflow, and stream temperature in the North Fork basin due to climate warming, we apply gridded meteorological surface data with a physically based hydrology model, the Distributed Hydrology Soil Vegetation Model (DHSVM), and a stream temperature model, the River Basin Model (RBM).

We establish the spatial characteristics of the North Fork basin at a 50 m grid resolution and apply historical meteorological gridded surface data developed by Linveh et al. (2013) to calibrate the DHSVM to streamflow from a USGS stream gauge near the mouth of the North Fork of the Stillaguamish. Field work was conducted in the summer of 2017 to determine stream morphology, discharge, and stream temperatures at a number of stream segments for the RBM calibration to a Washington Department of Ecology temperature gauge. Riparian input parameters for individual stream segments were characterized using a combination of LiDAR data, NOAA landcover data, and estimations based on previous Stillaguamish modeling studies. We simulate forecast climate change impacts using gridded downscaled data from ten global climate models of the CMIP5 with RCP4.5 and RCP8.5 forcing scenarios developed using the multivariate adaptive constructed analogs method (Abatzoglou and Brown, 2011). Simulation results project a trending increase in stream temperature into the 21st century as a result of higher air temperatures, deceasing snowpack, and lower spring and summer stream discharges. By examining relative temperature variance of North Fork Stillaguamish tributaries, management focus can be directed to locations where the most amount of impact from restoration is likely to occur.