Wildfires and Storm Events: Multiple Stressors Alter Stream Form and Function in Semi-Arid Watersheds

Climate change in the western U.S. is causing larger wildfires and more extreme precipitation events. When these two ecological changes collide, they create massive ecosystem disturbance, affecting terrestrial and aquatic environments as well as human well-being. In October 2018, such a scenario occurred when the remnants of Hurricane Rosa dumped torrential rain on a two-week old, 610-km2 burn scar in central Utah. The wildfires, flash flooding, and debris flows triggered the evacuation of approximately 10,000 residents and created a sediment plume in the downstream lake that was visible from space. We collected stream water samples from 10 watersheds during and after the storm, allowing us to quantify the interactive effects of megafire and extreme rain on aquatic biogeochemical fluxes. We analyzed samples for a broad suite of physicochemical parameters including organic matter concentration and biodegradability, water isotopes, major ions, trace metals, and nutrients. While the burned and unburned streams showed various concentration-discharge relationships, the effects of the megafire were apparent in nearly every parameter we quantified, increasing particulate loading and resulting in a substantial loss of terrestrial carbon, nitrogen, and phosphorus.