Hydrologic Alterations from Climate Change Inform Assessment of Ecological Risk to Pacific Salmon in Bristol Bay, Alaska

We developed an integrated hydrologic model of the upper Nushagak and Kvichak watersheds in the Bristol Bay region of southwestern Alaska, a region under substantial development pressure from large-scale copper mining. We incorporated climate change scenarios into this model to evaluate how hydrologic regimes and stream temperatures might change in a future climate, and to summarize indicators of hydrologic alteration that are relevant to salmon habitat ecology and life history. Model simulations project substantial changes in mean winter flow, peak flow dates, and water temperature by 2100. In particular, we find that annual hydrographs will no longer be dominated by a single spring thaw event, but will instead be characterized by numerous high flow events throughout the winter. Stream temperatures increase in all future scenarios, although these temperature increases are moderated relative to air temperatures by cool baseflow inputs during the summer months. Projected changes to flow and stream temperature could influence salmon through alterations in the suitability of spawning gravels, changes in the duration of incubation, increased growth during juvenile stages, and increased exposure to chronic and acute temperature stress. These climate-modulated changes represent a shifting baseline in salmon habitat quality and quantity in the future, and an important consideration to adequately assess the types and magnitude of risks associated with proposed large-scale mining in the region.