FLOODPLAIN STORAGE CAPACITY OF THE UPPER YAKIMA RIVER TRIBUTARIES, KITTITAS COUNTY, WA

Large wood (LW) restoration projects were recently implemented in the Upper Yakima Basin following the destructive logging practices of the early 20^th Century, which stripped Upper Yakima River tributaries of LW. The removal of LW increased incision, isolating channels from floodplain aquifers, and degrading resident and anadromous fish habitat. Returning streams to their natural state through LW additions is hypothesized to increase floodplain groundwater storage by decreasing channel incision, increasing floodplain-channel connectivity, and raising the water table elevation. Additional storage in floodplain aquifers can help combat the adverse effects of climate change, namely decreasing snowpack and earlier melting. Storing infiltrating snowmelt in shallow floodplain aquifers during peak spring runoff could allow the natural release of groundwater as baseflow later in the growing season. Taneum Creek, Indian Creek, and Teanaway River have established LW projects and are candidates for floodplain aquifer storage. However, a widespread blue-gray glacial lacustrine clay in these floodplains could affect potential aquifer transmissivity and storage capacity. We quantified the floodplain capacity considering a regional confining glacial clay layer to reveal the effects of glacial history on groundwater recharge, storage, and flow in shallow floodplain aquifers. We collected floodplain stratigraphy data and a maximum glacial clay elevation via exposed streambeds, auger cores and well logs. These data contributed to a refined volumetric storage capacity calculation for the floodplain aquifers. Grain sizes and infiltration rates through representative stratigraphic units further constrained the transmissivity properties of the shallow floodplain aquifers. The maximum elevation of the blue-gray clay was applied to surrounding tributaries to calculate realistic floodplain aquifer volumes in similar tributary watersheds. Gaining a better understanding of the floodplain stratigraphy in the upper tributaries permitted a refined aquifer capacity calculation and informed interaction between past glacial processes and current water storage solutions in the Yakima River Basin.