The Skagit watershed has a long history of conflict over water given the high demands on water for drinking, irrigation, hydropower and instream flows to protect and preserve the resources of the Skagit River including fish and wildlife. With climate change, summer streamflow is expected to decrease throughout the basin, which could increase conflicts over water for multiple uses at certain times of the year.

The Skagit Water Supply and Demand Synthesis was developed in response to a request by the Washington State Legislature Joint Task Force on Water Supply (Co-Chairs: Representative Debra Lekanoff and Senator Keith Wagoner). The Task Force includes representatives from multiple sectors (e.g. drinking water and agriculture), Tribes, Washington Department of Ecology and local governments engaged in water management in the Skagit basin. The Skagit Water Supply and Demand Synthesis is led by Jon Yoder, at Washington State University’s Water Research Center, with contributions from HDR Engineering and NOAA Northwest Fisheries Science Center.

This study advances our knowledge of water in the Skagit by synthesizing available data and information into a single resource. The synthesis enables better integration and direct comparison of different sources of water (groundwater and surface water) and different uses of water (irrigation, drinking water and instream flows for fish and habitat). As water managers and decision-makers identify ways to mitigate current and future water shortages, it is crucial for all parties involved to have a common understanding of current and future water conditions.

We synthesized data and literature to develop an ArcGIS map series, consisting of multiple story maps summarizing the dimensions of water use and supply in the Skagit basin — surface water, groundwater, fish habitat, flow regulation, agricultural demand and residential demand. Each story map describes what is known about current and future conditions (related to climate change and growth and development), and identifies key knowledge gaps for advancing our understanding of water use and needs across the basin.

The Surface Water story map summarizes current streamflow and water volume for the Skagit River mainstem and several tributaries using streamflow observations from USGS stream gauges. Projections of change in surface water are modeled with the Distributed Hydrologic Simulated Vegetation Model (DHSVM) using climate inputs based on a methodology developed by Bandaragoda et al. 2021.

Future streamflow projections are produced for two greenhouse gas scenarios and 10 global climate models. Results are presented for the mainstem Skagit River and multiple tributaries for changes in annual and summer streamflow (i.e.: surface water supply). Annual streamflow is important for total water availability for all uses and summer streamflow determines water availability at a critical time of low supply and high demand.

Photo credit: SounderBruce, CC BY-SA 4.0 , via Wikimedia Commons

Results from the surface water synthesis show that unregulated surface flows, or flows unmodified by hydropower dams and water withdrawals, are projected to decrease in summer. This trend is consistent among models and our certainty in this result is fairly high. In contrast, total annual unregulated surface flow at Mt. Vernon is projected to increase by three percent on average for 2020–2050 (relative to 1980–2010). However, certainty in this result is lower, as some models project increases, others project decreases and the magnitude of changes are within the range of historical variability. Surface water in July to September (late summer low flow period) is projected to decrease by up to 40 percent in the Sauk River and the upper Skagit River above Marblemount, whereas surface water in sub-basins in the lower Skagit River is projected to decrease but by less — only five to 20 percent for 2020–2050 (relative to 1980–2010). Decreases in late summer surface water are consistent among climate models and outside the range of historical variability in many tributaries, so certainty in this change is higher than for changes in annual surface water.

The current net effect of seasonal water supply and use patterns in the Skagit River basin is that the Skagit River flows are often below the flow levels of the 2001 Skagit Instream Flow Rule from late July through November, potentially causing stress for river-based ecosystem services and salmonid fisheries that the Instream Flow Rule was designed to protect. Projected streamflows suggest that even with current demands and flow management, summer streamflows will fall below the Instream Flow Rule levels more frequently, for longer periods, and farther below the target flows when they do.

Additional information on climate change effects throughout the Skagit Basin are available on the website of the Skagit Climate Science Consortium.