The Effects of Channel Incision and Land Use on Surface-Water/Groundwater Interactions in the Teanaway River Basin, Washington, USA

The Teanaway River basin, a major tributary to the Yakima River, is host to several restoration projects with the intention of returning the river channel to a more natural state and improving riparian habitat. These projects may also increase aquifer storage and potentially increase summertime streamflows. This study of the Teanaway Valley Family Farm, an 88-hectare parcel on the main-stem Teanaway River that was recently purchased by Washington Department of Fish and Wildlife, provides hydrogeologic data that will inform these restoration projects. Following the purchase of this land, ten wells were installed within and slightly above the floodplain in order to determine seasonal variations in groundwater level, in part to investigate the mortality of a cottonwood grove near the Teanaway River. Well cuttings revealed that the upper third of the floodplain in this region is underlain by a thick clay deposit above the bedrock, which limits the extent of river water interaction and reduces the overall aquifer storage potential. In this study, groundwater level and temperature data from pressure transducers for a 15-month period for all ten wells is analyzed and compared to river elevations. This data showed that there were two distinct “pulses” of groundwater recharge, occurring during warm spells in January and February 2020. Water elevation within the alluvial aquifer then declines over a 4-month period towards baseflow conditions so that during the summer months, groundwater in the cottonwood grove was at least two meters below the surface. Thus, the cottonwood grove does not have access to groundwater during the dry summer months. Stable isotope analyses were performed on the groundwater, soil water, and river water in order to determine the extent of river water infiltration into the unconfined alluvial aquifer adjacent to the river. The stable isotope composition of the river and well water from the four wells closest to the stream were nearly identical, whereas groundwater from three wells further up the floodplain appears to be a mixture of river water and pre-existing groundwater. Thus, the river naturally supplies water to the cottonwood grove area throughout the year. However, the stream incision and pumping from upstream ponds for irrigation has resulted in a lower water table within the floodplain during the dry summer/early fall months. The cottonwoods likely survived the challenges of the stream incision with a supply of water from prior irrigation practices that artificially raised the water table during the dry season