Wildfire Effects on High Elevation Watersheds – Trouble in the Headwaters

Wildfire is a natural disturbance that alters biogeochemical processes in terrestrial to aquatic ecosystems in many forest watersheds. The downstream consequences of wildfire mirror the extent and severity of a watershed burned and depending on the pace of vegetation recovery these can persist for decades. Recent increases in the occurrence of wildfires in higher elevation forests, where fires have been historically infrequent and post-disturbance recovery is typically slow portend important changes that have implications for clean water supply. In 2020, four large wildfires burned catchments that supply water to agricultural, industrial and residential users on both sides of the Continental Divide in Colorado and southern Wyoming. Here we evaluated wildfire ash and tracked seasonal and storm event patterns in stream water chemistry for three post-fire years after the Cameron Peak Fire, the largest in Colorado history. The first post-fire year, high-intensity rainstorms generated ‘black water events’ that transported nutrients contained in ash, charred organic material and sediment into headwater tributaries and impacted storage reservoirs and downstream water utilities. Consistent with transport via subsurface flowpaths, nitrate in streamwater increased significantly the second and third post-fire years after the fire, even as suspended sediment declined. Conversely, except during summer rainstorms, ortho-P returned to pre-fire, sub-detection levels during years 2 and 3. Elevated transport of nutrients from burned hillslopes to surface water compound existing stresses from chronic, elevated atmospheric N deposition and increasing temperatures. Together these increase the risk of harmful algal blooms and represent a potential long-term impact of severe wildfires.