As wildfires grow more frequent and intense with climate change, their hazards extend far beyond charred forests and razed properties. A new study from USGS researchers and collaborators titled “Watershed Sediment Yield Following the 2018 Carr Fire, Whiskeytown National Recreation Area, Northern California” looks at how wildfires can affect entire watersheds by clearing away groundcover, destabilizing hillsides, and jeopardizing water supplies.

One landscape-scale impact from wildfires is the increase in sediment. Intense wildfires consume vegetation and burn down into the soil, creating conditions that result in increased erosion: seared soil that doesn’t absorb rainwater and instead causes it to run downslope, picking up sediment along the way; less groundcover vegetation, which holds soil in place; and fewer trees, which reduces canopy cover and allows rain to impact the ground directly.

The researchers examined how one particularly destructive fire—the 2018 Carr Fire in northern California—created large amounts of sediment that flowed into a federally managed reservoir, Whiskeytown Lake, within Whiskeytown National Recreation Area. These sediments can reduce water quality, decrease space for water storage in reservoirs, and sometimes produce hazardous debris flows.

In the year after the Carr Fire, the researchers found that greater-than-average precipitation moved large sediment loads through the watersheds—in some cases, orders of magnitude more sediment compared to pre-fire amounts.

“This study involved a novel application of coastal and marine mapping techniques to characterize wildfire impacts,” said Amy East, a USGS research geologist with the Pacific Coastal and Marine Science Center and lead author of the study. “Research like this is becoming increasingly necessary as climate change increases wildfire risk, especially in places like northern California that were less flammable under cooler, wetter climate conditions.”  

Few data on fire response currently exist from this region.  Results from this study—conducted with support from federal partners including the National Park Service, which oversees Whiskeytown National Recreation Area—will aid in prediction of future fire response in northern California, said East.

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