DECADAL-SCALE EFFECTS OF LARGE WOOD RESTORATION ON CHANNEL MORPHOLOGY AND GROUNDWATER CONNECTIVITY, TANEUM CREEK, WA

The importance of large wood (LW) in creating channel complexity is widely recognized; however, few LW projects have been in place long enough to track meaningful channel changes on a decadal timescale. Taneum Creek, located in central Washington, is one of the earliest LW restoration areas (2008) in the Yakima River Basin and the central Cascade Mountains. The occurrence of a large flood in 2011 provided further channel change by mobilizing LW and channel sediments. Channel change was documented with ArcMap by digitizing channel sinuosity, braiding, channel migration using high-resolution satellite orthoimagery and National Agriculture Imagery Program imagery from 2006-2020 and complemented with airborne LiDAR (Light Detection and Ranging) imagery from 2010, 2014, and 2018. A greenness index derived from Landsat satellite imagery was used to assess the changes in floodplain vegetation over time as a proxy for groundwater-floodplain connectivity. In response to the large flood of 2011, LW created new channel complexity, such as braiding, sinuosity increases, side-channel formation, and increases in pool-riffle sequences. The reaches with increased channel complexity related to the LW and large flood also increased the connection between groundwater and the floodplain, by increasing side channel flow and allowing beaver dam construction off the main channel. The resulting increased area of interaction between the channel and floodplain is illustrated by increasing greenness indices. Groundwater-floodplain connectivity measurements taken using a conductivity probe and piezometers to identify areas of groundwater upwelling at the study reaches illustrate the potential for further study of groundwater channel influx. The combination of the investigation into groundwater and large wood, in combination with the LW response to the large flood provide a detailed picture into how decadal scale changes resulted from LW restoration. The result being increased channel complexity and groundwater-floodplain connectivity is important for maintaining diverse aquatic and riparian species.