Patterns in the timing and volume of precipitation and snowmelt are projected to shift due to climate change. These shifts will lead to changes in the timing of peak and low streamflows across the state. Specific, localized projections of future streamflow can help inform decisions regarding how to manage water resources and salmon habitat as the climate changes.

This project built off of previous climate modeling efforts to develop new projections of streamflow, temperature and sediment load along the Skagit River. Future projections were calculated using global climate models for multiple overlapping fifty-year periods starting from 2010 to 2099. To simulate streamflow, we used the Distributed Hydrology Soil Vegetation Model (DHSVM) – a coupled glacio-hydrology model. Validations and corrections to the glacio-hydrology model were conducted using empirical data (collected by North Cascades National Park), naturalized flows at reservoir locations (three reservoirs), and observed stream gauges (where and when available at 16 Skagit River tributaries). Mean annual suspended sediment load estimates were obtained using historical (1960-2010) and future modeled streamflows in a suspended sediment rating curve (Jaeger et al., 2017) for lower and middle Sauk (Darrington) locations.

Changing Streamflow in Icicle, Peshastin and Mission Creeks. The purpose of this project is to leverage existing hydrologic change datasets to estimate future changes in streamflow in Icicle, Peshastin, and Mission Creeks as well as the seven alpine lakes. These will be used to evaluate proposed alternatives for managing water in Icicle Creek.

Skagit Water Supply and Demand Synthesis. The aim of this project is to support effective water management in the Skagit Basin under climate change. This report provides the Joint Legislative Task Force on Water Supply with information about water supply and demand in the Skagit basin, including current and projected future data on surface water, ground water, residential demand, agricultural demand and instream flow needs for fish habitat. The UW Climate Impacts Group team developed the surface water supply component and contributed to the synthesis of information on flow regulation.

Photo credit: Walter Siegmund, CC BY-SA 3.0

This project was made possible in part by the Skagit Climate Science Consortium, a 501 c(3) nonprofit comprised of a group of member research scientists from federal, municipal, tribal, and university organizations who work in the Skagit basin.

Early support for the Skagit DHSVM modeling project (2015) has been funded in part by the Climate Change Research Initiative, Seattle City Light, a public utility that serves the City of Seattle (Appendix C and Appendix D).

Early support for this project has been funded in part by the United States Environmental Protection Agency under Assistance Agreement to the Swinomish Indian Tribal Community and to the Sauk-Suiattle Indian Tribe.

We acknowledge the World Climate Research Programme's Working Group on Coupled Modeling, which is responsible for CMIP, and we thank the climate modeling groups (listed in Section D of the report) for producing and making available their model output.

Special thanks is due to Jon Riedel for providing the North Cascades glacier mass observations, feedback and review over the course of the project; Andrew Fountain for providing historic glacier extent GIS data; Bart Nijssen for coordinating the use of meteorological data from the Integrated Scenarios Project, computing resources (2015), and review over the course of the project; Matt Stumbaugh and Alan Hamlet for providing streamflow projections from Skagit River Lowlands Low Flow Study (2015) to integrate with results from this report and for review of the study design and report.

Photo credit: Sounder Bruce, CC BY-SA 4.0