Slope erosion worsens in California due to wildfires and heavy rains

Over the past three decades, California has experienced increasing erosion after large wildfires, a phenomenon that not only endangers water resources and ecosystems but is also likely to worsen with climate change, researchers say.

A new study from the U.S. Geological Survey has documented a tenfold increase in hillside erosion after fires in Northern California from the late 1980s to the 2010s, with the majority of the largest sediment-producing fires occurring in the last decade.

This erosion creates many problems. When heavy rains hit 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 vulnerable to flash floods.

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

“In Northern California, we see a dramatic increase (in post-fire erosion) from the first to the second, and then the third and fourth decades,” said Helen Dow, a USGS research geologist and lead author of the study. “There’s a dramatic increase in sediment, both in terms of mass … and also in terms of yield, which is mass per area.”

By integrating detailed modeling and field observations, the research team quantified soil and sediment loads from erosion between 1984 and 2021 for each year following a large wildfire, which scientists classified as being larger than 25,000 acres. The method helped put a number on a problem that has long troubled environmentalists, forest managers and water conservationists.

“It’s not surprising … but it’s nice to see it quantified by the USGS,” said Glen Martin, a spokesman for the environmental nonprofit California Water Impact Network.

“It highlights the bigger problem, which is that California – its water supplies, its reservoirs, its fisheries are already on the brink, and these catastrophic fires are going to throw everything off course for a variety of reasons,” said Martin, who was not involved in the research.

Several studies have already shown that wildfires are becoming larger and more intense due to climate change. These same factors are also increasing the frequency of more extreme rainstorms across the state, leading to more “weather whiplash” events.

The USGS document, which was published in the Journal of Geophysical Researchfound that 57 percent of the state’s post-fire erosion occurred upstream of reservoirs, “indicating a growing risk to water security.” Reservoirs are a key part of the state’s fragile water system, but sediment influx can both decrease a basin’s capacity and degrade its water quality.

“These results indicate increasing pressure on water resources due to post-fire erosion and ongoing climate change,” the study authors write.

Martin said the increased erosion is part of a “vicious cycle” of more fires, more eroded soil, 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 expect erosion after wildfires will only increase statewide without comprehensive mitigation efforts, but Dow said documenting the extent of the problem is an important step for state and federal officials to seek interventions.

“Knowing that this is a growing problem in Northern California and having a sense of the magnitude of the problem in both Northern and Southern California could inform how agencies think about fires,” Dow said.

“What we need to do is strengthen fuel controls on public and private lands,” Martin said, “so that when fires do occur, they’re not absolutely devastating, they’re not burning down to the mineral soil and turning the landscape into a moonscape.”

Martin said fuel control could include prescribed burns and mechanical thinning, or targeted removal of certain trees.

California is already aware of the devastating and widespread effects that erosion and debris flows can have following large fires.

In Montecito, heavy rains after the Thomas Fire in 2018 triggered an avalanche of mud and debris that ravaged the town, killing 23 people and destroying 130 homes.

In 2022, a massive fish kill occurred in the Klamath River after successive landslides dumped fire-scarred soil and debris into the watershed, causing dissolved oxygen levels to drop for several hours.

The sediment buildup has also affected the Devil’s Gate Dam in Pasadena. Excessive erosion has also clogged countless culverts, blocked roads and buried infrastructure, increasing flood and safety risks.

This year, the park fire, which ravaged Mill and Deer creeks in the Central Valley, threatened what Martin called some of the last strongholds of endangered spring chinook salmon. He said the heavy rains could undermine the progress wildlife officials have made to sustain the fish population.

“Our salmon are already decimated by excessive water diversions; add this to that and it becomes virtually impossible for these salmon to come back,” Martin said.

Dow said the team’s research only considered erosion in the first year after a wildfire, which likely underestimates the extent of the problem.

The study was released at the same time as another USGS study that measured sediment in the Carmel River along California’s central coast. It found that after wildfires and extreme rains, sediment in the watershed increased significantly compared to long-term averages.

As the state faces a host of other major water-related issues, combating post-fire erosion through better land conservation and forest management is necessary, Martin said, but not easy.

“It’s going to take time, and it’s more than that: it’s going to take a lot of public will and money,” he said. “This is a crisis situation. … The situation is only going to get worse until we take this issue seriously.”

Los Angeles Times 2024. Distributed by Tribune Content Agency, LLC.