Over the past three decades, California has seen increased erosion following major wildfires, a phenomenon that not only threatens water resources and ecosystems, but is also likely to worsen as climate change continues, scientists say.

A new study by the U.S. Geological Survey has found a tenfold increase in hillside erosion following wildfires in Northern California from the late 1980s to 2010, with most of the largest, sediment-producing fires occurring in the past decade.

This erosion causes a range of problems. When heavy rains scour the charred hillsides, debris flows can clog rivers and streams, depriving fish of oxygen. Sediment runoff can also fill reservoirs and take up valuable water storage space, damage flood-control infrastructure and threaten nearby communities prone to flash flooding.

The research team noted that erosion from wildfires has accelerated across the state since 1984, with the most noticeable change in the northern half of the state.

“In Northern California, we really see this huge increase (in post-fire erosion) from the first decade to the second, third and fourth,” said Helen Dow, a USGS research geologist and lead author of the study. “There’s just been a huge increase in sediment, both in mass … and when you look at the yield, which is mass per area.”

By incorporating detailed modeling and field observations, the research team quantified soil and sediment erosion loads from 1984 to 2021 for each year following a major wildfire, which scientists classify as larger than 25,000 acres, providing insight into a question that has long been on the minds of ecologists, forest managers, and water conservationists.

“It’s not surprising, but it’s good that the USGS quantified it,” said Glen Martin, spokesman for the California Water Impact Network, a nonprofit environmental group.

“It shows that there is a bigger problem, and that is California — your water resources, your reservoirs, your fisheries are already at the brink of depletion, and these catastrophic fires are going to make that worse for a variety of reasons,” said Martin, who was not involved in the study.

Several studies have already shown that wildfires are becoming larger and more intense due to climate change. Those same forces are also increasing the frequency of more extreme rain storms across the state, leading to more episodes of “whiplash.”

The USGS article that was published in Journal of Geophysical Researchfound that 57% of post-fire erosion in the state occurred above reservoirs, “indicating an increasing risk to water security.” Reservoirs are a key part of the state’s fragile water system, but sediment inflow can both reduce reservoir capacity and degrade water quality.

“These results indicate increasing pressure on water resources resulting from post-fire erosion as climate change progresses,” the study authors write.

Martin called the increase in erosion part of an “unhealthy cycle” of more fires, more soil erosion, which leads to more infrastructure failures and, ultimately, less water.

“This has huge implications for everything from fisheries to water supplies, and this study confirms that,” Martin said.

USGS researchers predict that without comprehensive mitigation efforts, wildfire erosion will only worsen statewide. But Dow said documenting the scale of the problem is an important step for state and federal officials to look for ways to intervene.

“Knowing that this problem is getting worse in Northern California and knowing the scale of the problem in both Northern and Southern California can help agencies understand how they think about wildfires,” Dow said.

“We need to increase fuel control on both public and private lands,” Martin said, “so that the fires are not absolutely devastating, burning mineral soils, turning the landscape into a lunar landscape.”

Martin said fuel control could include controlled burning and mechanical thinning or the intentional removal of some trees.

California officials recognize the devastating and widespread impacts that large wildfires can cause due to erosion and debris flows.

In Montecito, heavy rains following the 2018 Thomas Fire triggered an avalanche of mud and debris that devastated the city, killing 23 people and destroying 130 homes.

In 2022, the Klamath River experienced a mass fish die-off after successive landslides dumped fire-damaged soil and debris into the watershed, causing dissolved oxygen levels to drop for several hours.

Sediment buildup has also plagued the Devil’s Gate Dam in Pasadena. Excessive erosion has also clogged countless culverts, blocked roads and buried infrastructure, raising flooding and safety concerns.

This year, a park fire that tore through Mill and Deer creeks in the Central Valley has threatened what Martin called the last strongholds of endangered Chinook salmon. He said heavy rains could undermine progress wildlife officials have made in maintaining fish populations.

“Our salmon are already decimated by over-drainage; when you add this on top of that, it basically makes it almost impossible for those fisheries to come back,” Martin said.

Dow said the team’s study only takes into account erosion that occurs in the first year after a wildfire, so it likely underestimates the full scale of the problem.

The study was published alongside another USGS study that measured sediment in the Carmel River along California’s Central Coast. It found that following wildfires and extreme precipitation events, sediment in the watershed increased significantly compared to long-term averages.

Martin said that given the range of other serious water issues, combating post-fire erosion through better land protection and forest management is essential, but it is not easy.

“It will take time, and even more: it will require a lot of public will and money,” he said. “This is an emergency. … It will only get worse until we start dealing with this seriously.”

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