POST-WILDFIRE SEDIMENT CASCADES: WATERSHED-SCALE DYNAMICS AND THE VULNERABILITY OF RESERVOIRS (Invited Presentation)

Over the past three decades, the area, frequency, and severity of wildfires have rapidly increased. These trends are expected to continue into the future due to climate warming, and in some ecosystems, artificially high fuel loads. While wildfires frequently cause immediate, short-term impacts to water quality, the excessive erosion after severe burns can result in catastrophic debris flows that deliver significant amounts of sediment to rivers. Consequently, the continuing rise in wildfire activity is projected to increase sediment yields in the majority of western watersheds. As this sediment is transported downstream through river networks, it could accelerate reductions in the long-term water storage capacity of western reservoirs now relied upon by tens of millions of people. Here we present a new modeling framework capable of identifying and quantifying the risk that wildfire and subsequent erosion may pose to downstream water infrastructure. This framework links a fuels-dependent wildfire risk model with models of post-wildfire debris flow generation, sediment delivery, and network-scale sediment routing to track the cascade of post-wildfire sediment through watersheds. Our initial application of this modeling framework is focused on quantifying the wildfire-sedimentation risk to reservoirs serving Salt Lake City, Utah. While the watersheds that drain to these reservoirs have not experienced serious wildfire in recent history, ignitions in this populated area are frequent and this region has experienced an increase in large, high-severity fires. Compiling results from multiple simulations of wildfire and post-fire storm characteristics, we quantify the vulnerability of each reservoir as the predicted range in percentage loss of constructed storage capacity. This modeling approach allows for assessing the combined effects of sediment generation, delivery, and connectivity at the scale of large watersheds and over decadal timescales. In addition to providing valuable insights for the pyrogeomorphic community regarding post-wildfire sediment dynamics, identifying watersheds that are susceptible to catastrophic post-wildfire erosion and functionally connected to downstream reservoirs will be valuable to the general public, as well as fire, forest, and water managers.