RAPID: Downstream water quality impacts of post-wildfire debris flows

Wildfires are increasing in extent, severity, and frequency due to climate change. Wildfires alter physical and chemical properties of the soil surface, remove canopy cover, and reduce the stability of hillslopes. Mid- to high- intensity rain events on these highly altered landscapes trigger debris flows on steep slopes, which are common across the Western United States. These debris flows remove material from hillslopes and headwater channels, depositing it in larger streams and rivers. Studies have examined the impact of regular debris flows on water quality, particularly on sediment loading, however post-fire debris flow material has a unique chemical signature due to being recently burned. In the absence of debris flows, wildfires can degrade water quality, specifically increasing nutrient concentrations, sediment and salt loads, and change the quality of dissolved organic carbon, which can impact aquatic ecosystems and drinking water supplies. Post-fire debris flows likely amplify water quality impacts to watersheds that recently experienced a wildfire. However, there is a lack of data and understanding of how post-fire debris flows impact water quality, partially because they occur very soon after a fire, making them very difficult to study. This RAPID project will collect a comprehensive dataset to document the water quality impacts associated with debris flows occurring in the immediate aftermath of a large, mid- and high-elevation fire in summer 2024. The perishable dataset associated with this research will catalyze new research aimed at advancing mechanistic understanding and prediction of changing fire and climate regimes on water quality in the western US – a historically understudied and increasingly urgent line of inquiry. The investigators will conduct this research in and downstream of the Wapiti Fire burn perimeter, that encompasses the headwaters of the south fork of the Payette River watershed in central Idaho. This area is representative of much of the forested areas in the northern Rocky Mountains, which are experiencing larger, more frequent, and higher intensity fires. The project team will collect perishable water quality, soil nutrient, and geospatial data. Soil characterization across a range of burn severity and debris flow materials will support interpretation of water quality data, as will geospatial characterization of the watershed, fire, and debris flow inputs to the river. The data can also be used by researchers and land managers to build regional models predicting the location and likelihood or debris flows – something that poses a risk to infrastructure and human health in addition to ecosystem health. The researchers will collaborate with other university and federal researchers to expand understanding of post-fire effects on snow stability and the aquatic ecosystem. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.