After wildfires, areas like this are more prone to landslides when wet atmospheric rivers batter the area. Photo Richard Bednarski / Sierra Nevada Ally

SIERRA NEVADA ALLY: After wildfires, wet storms and burn scars join forces in elevating landslide risk

Atmospheric rivers are here to stay. What impact do they have on fire stricken landscapes?

By Richard Bednarski, Sierra Nevada Ally

This story was produced by the Sierra Nevada Ally, a nonprofit, nonpartisan news outlet focused on civics, climate, and culture.

The Feather River Canyon begins just outside of Quincy, California. This corridor is home to Highway 70, one of the few year-round arteries to the many mountain communities. It is known for fishing and notable whitewater rafting, as well as steep canyon walls.

The holiday season brought a series of storms to this area as part of an atmospheric river, a long, narrow plume of moisture that can stretch for thousands of miles. Think of it as a rain storm that travels along the jet stream. These events not only bring increased rainfall and provide much-needed precipitation to the American West; they also bring increased risks of landslides, especially in areas affected by wildfires.

Since the 2021 Dixie Fire, there have been nearly a dozen reported landslides that have closed Highway 70 along the Feather River Canyon. These slides have ranged from small to major, closing the highway from just a few hours to days or weeks at a time. The trifecta of wildfires, topography, and heavy rainfall work effectively to dislodge the exposed rock and dirt.

“When you take away the vegetation, the raindrop goes directly to the soil and that mechanical action of the raindrop on the soil can help dislodge soil particles and start moving material down slope,” said Nina Oakley, Ph.D, an applied meteorologist and climatologist for the California Geological Survey.

Oakley specializes in atmospheric rivers and geohazards, and her work focuses on the interaction between extreme rain events and post-fire burn areas. She stressed that these burned areas have a huge affect on future landslide activity, because without vegetation on the surface, raindrops have a stronger impact force on the ground, which can more easily dislodge soil.

Though not directly related, atmospheric rivers and wildfires play an important role in the landscapes of California, Nevada, and other western states. Research has shown that as temperatures rise due to climate change, the atmosphere holds more water vapor and therefore injects more moisture with every rainfall. In fact, for every 1˚ Celsius increase in temperature, the atmosphere can hold 7% more water vapor. In burn areas, like those in eastern California, these elevated moisture levels are leading to landslides.

A rockslide near Rusk Creek in January 2023 closed Highway 70 for several days. Photo courtesy Caltrans.

These events in turn lead to increasing road maintenance work to combat the effects to travel corridors in the region.

“Especially the Feather River Canyon, with the geography that’s in there, you get the water that comes through the rock with a lot of storms, so it definitely is prone to debris and slides,” explained Chris Woodward, spokesperson for Caltrans District Two.

Woodward, who has been with Caltrans for eight years, said there has always been land and rock slide activity in the canyon–but it’s increasing.

“It feels like since the Dixie fire, we’ve had even more [landslides], with a large portion of it being in the burn scar as well,” he said.

A previous landslide that has been cleared by Caltrans. A K-rail remains to stop and slow any debris that may move during wet weather. Photo Richard Bednarski / Sierra Nevada Ally

Atmospheric rivers and wildfires aren’t the only culprits causing landslides in the region. The unique Sierra Nevada geology and topography also contribute. Younger volcanic rocks lie on top of older granite stone, giving the Feather River Canyon a unique geology that is more prone to landslides. Throughout the canyon, there are already varying degrees of erosion that can be seen in some places.

Caltrans regularly patrols the Highway 70 corridor and makes minor repairs throughout the year, and the agency schedules extra crew members during winter storms. Woodward said the state divides Caltrans into multiple districts, allowing each unit to adequately focus on its assigned region. District Two maintains more than 4,000 miles of highway across eight counties. They have 22 maintenance stations and more than 30 different crews to cover this stretch of northern California.

“We also have staff, especially during storms, in areas where we’ve seen debris,” said Woodward. “The Feather River Canyon is a good example. You’re always getting some kind of debris, especially if you’re having heavier systems, a lot of rain, different things like that.”

Sometimes the slides and debris can be cleared by patrolling maintenance crews. Other times the slide is massive and the state sends it out to a contractor. Known as a director’s order, this allows Caltrans employees to continue to monitor the road while a contractor clears the slide.

A Caltrans plow patrolling the Feather River Canyon during the Atmospheric River system on December 26, 2024. Photo Richard Bednarski / Sierra Nevada Ally

How do atmospheric rivers play into this?

An atmospheric river “is a concentrated corridor of heavy vapor that leads to precipitation, and often heavy precipitation in California,” said Kristen Guirguis, Ph.D., a scientist with the University of California, San Diego.

Imagine a river with intermittent rapids traveling from the sub-tropics to the dryer mid-latitudes. Each series of waterfalls is a storm. Now imagine this river is more than 1,000 miles long and several hundred miles wide. With each waterfall, a storm pounds the West Coast, bringing rain and snow to the region. This is essentially what an atmospheric river resembles. However, not all atmospheric rivers are the same.

“There’s been research that suggests that in the future, precipitation is going to become more volatile,” said Guirguis. This translates to more dry spells in between storms followed by wetter, more intense storms.

“Under climate change, when you have a warmer atmosphere, then you can hold more water vapor. So there’s a potential for just wetter, wetter storms,” she said.

Atmospheric rivers are being increasingly recognized for their importance. Guirguis said they are important to study because these events deliver so much of California’s water, up to 50% percent in some locations. According to the American Meteorological Society, atmospheric rivers transport on average “more than double the flow of the Amazon River.”

It’s only been in  the last two decades that scientists, like Nina Oakley, have really begun to focus more on atmospheric rivers.

“We really need to consider those moisture transport processes to study precipitation extremes,” said Oakley. She added it is important to understand the mechanics of an atmospheric river and how no two are alike.

In 2023, the American Geophysical Union developed a five level scale for these potent storm systems. Similar to a hurricane rating, the intensity scale goes from AR-1 to AR-5. With AR-5 being the most intense, the rating is based on moisture content and duration. These two data points help meteorologists predict the potential rainfall amounts and impact to a region when forecasting an atmospheric river event.

The intensity of an atmospheric river depends on how long it lasts (typically 24 to 72 hours; horizontal axis) and how much moisture it moves over one meter each second (measured in kilograms per meter per second; vertical axis). While weaker atmospheric rivers can deliver much-needed rain, more intense storms are more damaging and dangerous than helpful. Credit: AGU, after Ralph et al. (2019).

But it’s not moisture alone that leads to landslides in a burn area.

“What will cause flash flooding and debris flows on a recent burn area is tied to rainfall intensity,” said Oakley. “We also have changes in the water repellency of the soil. These physical and chemical processes change the soil water repellency and the potential for erosion.”

Oakley is currently studying the burned area of California’s fourth largest fire, the Park Fire, which began outside of Chico, Calif. and burned almost 430,000 acres. That burn area acts as a valuable case study for scientists.

“The late November storm on the Park fire got about 10 inches of rain, but it was all kind of moderate intensity,” said Oakley. “So we didn’t have any debris flows observed there, even with all that rain, because it wasn’t intense.”

Without high-energy, intense rainfall, Oakley said an atmospheric river may not cause a landslide–but those aren’t the only concerns. Wildfires and atmospheric rivers can also cause increased sediment runoff, which alters waterways, fisheries, and water resource infrastructure. This intersection of rain and fire is blending the impact of climate change in compounding ways.

“There’s all these cascading effects of having a wildfire and rainfall occurring on it,” explained Oakley.

A vehicle driving past a previous landslide that is blocked with a K-rail on December 26, 2024. Caltrans uses these concrete barriers to stop the movement of debris. Once the pile reaches the height of the wall, crews remove the sediment. Photo Richard Bednarski / Sierra Nevada Ally

With winter in full swing and wildfires extinguished, more atmospheric rivers are on their way. How they impact the burn scars of the Caldor, Dixie, or Park fire is yet to be known. But scientists like Oakley and Guirguis are working to help inform the public about the potential impact of these weather events.

The best thing Oakley urged the public to do ahead of a storm is to stay informed.

“Tune into your local National Weather Service office and determine whether there’s a hazard in your burn area,” Oakley said. “Is it an extreme atmospheric river or not?”