128-11 The Geomorphology and Hydrology of Pre-Restoration West Fork Teanaway River, Kittitas County, WA

Abstract:

The West Fork Teanaway (WFT) River in Kittitas County, WA is an alpine river with an entrenched bedrock river channel, disturbed floodplain geomorphology, and degraded riparian ecology. Two stream restoration approaches, “stage 0” valley bottom reset and engineered logjam installation, will be implemented at the WFT in 2026-2027 by collaborators at the Washington State Department of Fish and Wildlife and Mid-Columbia Fisheries Enhancement Group. The objectives of this study were to constrain the pre-restoration geomorphic and hydrologic conditions of the WFT, including the shallow floodplain aquifer stratigraphy, the hydrologic regime of the WFT River, and the groundwater/surface water interactions in the shallow floodplain aquifer. Monitoring of groundwater and surface-water levels with eleven floodplain piezometers and two in-stream stilling wells began in November 2024 and ended in May 2026. The timing of floodplain groundwater recharge, baseflow recession, and surface water-groundwater mixing were determined through analysis of stable isotopes (δ¹⁸O and δ²H) and hydrographs of groundwater and surface water. The floodplain aquifer began baseflow recession into the WFT River by early June 2025 and ended by October 2025. Recharge of the WFT floodplain aquifer is sourced by surface water, and recharge timing occurred from October 2025 to mid-March 2026. The floodplain aquifer groundwater had a rapid, synchronous response to WFT River surface water, demonstrated by the hydrographs of the floodplain piezometers and stable isotope trends of surface water and groundwater. This relationship is most likely enabled by the coarse-grained stratigraphy providing preferential groundwater flowpaths in the floodplain aquifer. Alteration of the floodplain stratigraphy during restoration could impact this connection. These results will be used to determine hydrologic changes as a result from the two restoration approaches through future, post-restoration monitoring of the WFT study area

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