Skagit River Basin Groundwater Study

Overview

 

The Skagit River Basin Groundwater Study was a follow-up to the Skagit Basin Water Supply and Demand Analysis.

Western Washington University (WWU) and HDR Engineering, Inc. (HDR), in collaboration with the State of Washington Water Research Center, conducted a study to unravel some of the complexity of groundwater resources with a focus on the lower Skagit River valley, between Sedro-Woolley and Cape Horn. The study was designed to characterize subsurface hydrogeologic units and groundwater resources, including investigation of groundwater/surface water interactions and evaluate basin-wide groundwater baseflow (groundwater discharge flowing to streams).

The study aimed to produce a more complete understanding of the Skagit’s groundwater resources and aquifers to help water resource managers with decision making.  The study can also help well owners better understand how the use of groundwater is related to streamflow and how they have the potential to influence streamflow and aquatic habitat in their watershed.

Deliverables will include:

• Well log
• Three hydrogeologic cross-sections
• Memorandum describing the utilized data, methodology, and discussion of findings
• ArcGIS 3D hydrogeology framework model of the study area (electronic file)

 

Key Findings from the Skagit Basin Groundwater Study

 

• Aquifer characterization (Tasks 200 and 500) suggests the area is made up of complex geology. Data and monitoring validate aquifer continuity from the glacial outwash deposits in the eastern portion of the study area recharging Skagit floodplain aquifers and the Skagit River. There may be no truly isolated aquifers in the upland area that would serve as an alternative water supply.

 

• The evaluation of groundwater/ surface water interactions indicate that both Grandy and Muddy Creeks gain flow from groundwater in their respective watersheds and then discharge flow to the aquifer associated with the Skagit River floodplain during the low-flow period. Both Grandy Creek and Muddy Creek gain flow from groundwater discharge from outwash deposits. Grandy gains more flow. Differences in groundwater gains were due to the presence and thickness of glacial outwash. Both streams lose flow on the Skagit River floodplain because of seepage into the alluvial aquifer.

 

• Hydrograph separation efforts (Task 300) used 44 gaging stations of streamflow data collected between 1908 and 2021. From these and other data, this work offers a wide set of water balance data, from precipitation to basin runoff, including components partitioned into GW recharge, and streamflow, and the portion of streamflow moving through the GW system and emerging in streams as baseflow. Findings indicate that about 85 percent of the subbasins analyzed have estimated GW discharge rates that are higher than the GW recharge rates derived from the Synthesis Study (Yoder et al. 2021).

 

Report Summary, Submitted to the Washington State Joint Legislative Task Force on Water Supply 30 June 2025 

Skagit River Basin Hydrologic Study Reports:The Estuary Study, the Groundwater Study, and the Tributary Study

Related Research

Hydrologic Study of the Skagit Estuary

Skagit River Tributary Instream Flow Habitat Assessment