A Total Watershed Workflow For Assessing and Monitoring Post-Wildfire Impacts On Reservoir Water Quality

The US Army Engineer Research and Development Center (ERDC) Post-Wildfire Water Quality and Ecology team has initiated a study on watershed impacts to reservoir water quality following wildfire and other land disturbances. Wildfires and other rapid land use changes have the potential to impact downstream water quantity and quality for many years following disturbance events. Modeling tools are needed to develop guidance for water quantity and quality management for decision makers to maintain authorized reservoir uses and sensitive habitats during fire recovery. This paper will describe a workflow applied to examine water quality perturbations at Detroit Lake, Willamette National Forest, observed following the 2020 Beachie Creek and Lionshead fires which impacted nearly 50% of the contributing area to the lake. Initial analysis focuses on watershed water temperature, which is especially important since the streams studied are historically home to spring-run Chinook salmon. The workflow includes statistical analysis of historical data collected on major tributaries to the lake which show that although there are long term trends in the system, wildfires introduce statistically significant changes in discharge and water temperature. Additional post-fire field observations on previously ungauged and unmonitored tributaries are strategically paired with geospatial information to identify vulnerable sub-watersheds. Finally, ongoing monitoring will be incorporated to predict recovery timelines and improve the capability of watershed modeling products to capture the impacts of intensive land use change on water quantity and quality. The ultimate goal of this work is to enable efficient post-fire response and the selection of priority restoration efforts.