ASSESSING THE EFFECT OF IN-STREAM LARGE WOOD JAMS ON AQUIFER RECHARGE AT INDIAN CREEK, KITTITAS COUNTY, WASHINGTON

Numerous stream restoration projects in the Yakima River Basin in Washington have placed large wood (LW) into tributary channels. One intended effect is to divert water onto floodplains to increase groundwater (GW) recharge and seasonal storage in shallow alluvial aquifers during spring high flows to release GW into streams during the drier summer months. Large wood was emplaced in Indian Creek in Kittitas County, Washington in 2016. Potential changes in the groundwater recharge in the adjacent floodplain before and after the LW installation were investigated through stratigraphic analysis, stream-flow modeling, stream discharge, and GW levels in seven piezometers installed in 2014 and 2018. Stratigraphic descriptions of the stream banks reveal a ubiquitous clay/silt dominant layer (60-90 cm thick) at a depth of 1 meter or less, overlying a sand and gravel layer (15-50 cm thick), a second clay/silt layer (~30 cm thick), and another sand and gravel layer. These relatively continuous clay layers extend at least 2.2 km upstream from the mouth of Indian Creek on both sides of the channel. Similar clay units have been mapped in the region as glacial drift or lacustrine deposits. A 1-dimensional hydraulic model was run with and without channel obstructions at spring monthly average discharge and peak discharge. Modeling suggests the water surface elevation may increase ~10-50 cm within and upstream of LW. The stream flow and GW data before and after the LW emplacement show no detectable effect on seasonal or longer-term GW levels. Data loggers show that GW levels return to baseflow within a few days after precipitation events, suggesting GW flow within the permeable sand and gravel layers beneath or between the clay/silt layers. The potential for GW recharge and storage from infiltration of surface water due to channel changes following LW emplacement appears restricted due to the clay layers within the floodplain stratigraphy. This assessment of stratigraphy coupled with GW data and stream-flow model can provide insight into the effectiveness of GW recharge from LW restoration projects in the region or similar settings.