Where and When Does Streamflow Regulation Significantly Affect Climate Change Outcomes in the Columbia River Basin?

Abstract

The Columbia River basin is a large transboundary basin located in the Pacific Northwest, straddling the US-Canadian border. The basin spans seven US states and one Canadian province, encompassing a diverse range of hydroclimates. Strong seasonality and complex topography, combined with the prominent role of snow in the hydrologic cycle, give rise to spatially heterogeneous climate impacts on unregulated streamflow. The basin’s water resources are economically critical for the region, and regulation across the domain is extensive. This study investigates where and when regulation significantly affects projected changes in streamflow due to climate change by comparing climate outcomes across 80-member ensembles of unregulated and regulated streamflow projections at 75 sites across the Columbia River basin. Unregulated daily streamflow projections are taken from an existing dataset of climate projections. Regulated streamflow projections were modeled by the US Army Corps of Engineers and the US Bureau of Reclamation by hydro-regulation models that simulate system operations based on current and historical water demands. Regulation dampens large shifts in winter and summer streamflow volumes and cool-season high flow extremes. Results for changes in warm-season high flow extremes and dry-season low flow extremes are spatially variable. At historically snow dominant headwater reservoirs, regulation amplifies the change in warm-season high-flow extremes, but these effects generally diminish downstream where, in some cases, dampening effects occur. Regulation dampens dry-season low flow changes in headwater tributaries where regulation is large, but elsewhere regulation has little effect on changes in dry-season low flows.