Glacier Recession and the Response of Summer Streamflow in the Pacific Northwest United States, 1960-2099

The Pacific Northwest is the most highly glacierized region in the conterminous United States (858 glaciers; 466 km(2)). These glaciers have displayed ubiquitous patterns of retreat since the 1980s mostly in response to warming air temperatures. Glacier melt provides water for downstream uses including agricultural water supply, hydroelectric power generation, and for ecological systems adapted to cold reliable streamflow. While changes in glacier area have been studied within the region over an extended period of time, the hydrologic consequences of these changes are not well defined. We applied a high-resolution glacio-hydrological model to predict glacier mass balance, glacier area, and river discharge for the period 1960-2099. Six river basins across the region were modeled to characterize the regional hydrological response to glacier change. Using these results, we generalized past and future glacier area change and discharge across the entire Pacific Northwest using a k-means cluster analysis. Results show that the rate of regional glacier recession will increase, but the runoff from glacier melt and its relative contribution to streamflow display both positive and negative trends. In high-elevation river basins enhanced glacier melt will buffer strong declines in seasonal snowpack and decreased late summer streamflow, before the glaciers become too small to support streamflow at historic levels later in the 21st century. Conversely, in lower-elevation basins, smaller snowpack and the shrinkage of small glaciers result in continued reductions in summer streamflow.