Soil erosion: the silent risk between fires and floods

In a “first-of-its-kind study” U.S. Geological Survey (USGS) scientists quantified erosion for one year following every large fire in California from 1984 to the 2021. They documented a tenfold increase in post-fire hillside erosion. The majority of the largest sediment-producing fires occurring in the last decade. The team also noted that Northern California has experienced the most noticeable change.

Erosion causes a number of problems. Heavy rains on burn scars can cause debris flows, causing new hazards for structures or drinking water sources. Sediment runoff can fill up reservoirs, limiting the capacity for water storage and flood control.

Helen Dow, a research geologist led the team who incorporated detailed modeling and field-based observations of soil and sediment loads. They classified a “large wildfire” as having burned at least 25,000 acres. They found that 57% of post-fire erosion occurred upstream of reservoirs — “a growing risk to water security.” Reservoirs are key infrastructure for freshwater storage, flood risk mitigation, hydropower generation, and recreation.

The USGS researchers expect that erosion after wildfires will continue to increase without comprehensive mitigation efforts.

As the study’s lead author, Dow said that documenting the extent of the problem is an important step for state and federal officials to look for interventions. The findings have put a figure on a challenge that has long confronted ecologists, forest managers and water conservationists.

Another USGS study measured post-fire sediment yield, testing the WEPPcloud / wepppy framework for the Water Erosion Prediction Project (WEPP) model. Those researchers concluded that after wildfires and extreme rain, sediment in the watershed greatly increased — by a factor of 3.5 to 4.6 — compared with long-term averages.

“Knowing this is a problem that’s worsening … and having an idea of the size of the problem … might inform how agencies think about fire,” commented Dow. She added that the research “only took into account erosion within the first year after a wildfire, so it probably underestimates the full extent of the problem.”

Read the full study in the Journal of Geophysical Research.