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

The South Fork of the Stillaguamish River in Northwest Washington State is an important regional water resource and habitat for several threatened salmonid species. The river is currently subject to a temperature total maximum daily load, so it is important to understand how forecasted climate change will affect future stream temperatures and thus salmon populations and migrations. The South Fork basin is approximately 660 square kilometers with elevations reaching just over 2000 meters. Snowpack is the main contributor to spring and summer streamflow and currently serves as a stream temperature buffer. Previous modeling studies of western Cascade river basins predict that forecasted climate warming will result in a reduced snowpack and a lower spring runoff. To predict changes in hydrology and stream temperature in the South Fork Stillaguamish River, we used gridded meteorological data in the Distributed Hydrology Soil Vegetation Model (DHSVM) and the River Basin Model (RBM).

We establish the spatial characteristics of the South 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 Washington State Department of Ecology (Ecology) stream gauge. Field work was conducted in the summer and fall of 2018 to determine stream morphology, discharge, and temperature at ten stream sites throughout the basin. Data collected from field work were used in the calibration of the RBM to stream temperature data from the Ecology stream gauge. We used the calibrated models to simulate the effects of climate warming into the 21^st century 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, 2013). Forecast modeling indicates that the South Fork watershed will transition from a snow- to a rain-dominated basin into the 21st century resulting in a reduced snowpack and higher winter streamflows. Stream temperatures are projected to rise into the 21st century as a result of higher air temperatures, declining spring snowmelt, and lower summer streamflows.