Modeling the Effects of Climate Variability on Hydrology and Stream Temperatures in the North Fork of the Stillaguamish River

The Stillaguamish River in northwest Washington State, USA, provides water resources to local agriculture, industry and First Nations Tribes, and provides crucial habitat for several endangered species of salmonids. The watershed experiences a mild maritime climate and high relief, with rain and snowmelt dominating the streamflow. In anticipation of shifts in snowpack, streamflow, and stream temperature, I use projected global climate scenarios and numerical models to examine future climatic variability on streamflow and stream temperatures in the snow-melt dominated North Fork of the Stillaguamish River. I calibrated the physically based Distributed Hydrology Soil Vegetation Model (DHSVM) and River Basin Model (RBM) to gridded historical meteorological data in the basin and then applied downscaled, gridded projected climate data to predict streamflow and stream temperature changes through 2090 in this basin.Forecast modeling indicates that the North Fork watershed will transition from a snow- to rain-dominated basin into the 21st century as a result of increasing air temperatures. More precipitation in the winter will fall as rain rather than snow, resulting in up to a 43% increase in streamflow and a 56% decline in basin-wide snowpack. The reduced snowpack will melt out earlier and cause a decrease in spring and summer streamflow. Simulations of stream temperature indicate rising temperatures in every stream segment in the basin by the end of the 21st century as a result of higher air temperatures, declining snowpack, and lower summer streamflow. Monthly average stream temperatures could increase by up to 7.4 oC. In addition, the temperature thresholds for every life cycle of endangered salmon species are increasingly exceeded through time, putting at risk already endangered salmon species. By the end of the 21st century, the main stem may experience up to a 10-fold increase in number of days per year exceeding salmon temperature thresholds. Reach-scale predictions of stream temperature trends through the basin offer water resource managers a tool for focusing riparian and groundwater restoration efforts.