Stanislaus National Forest, California, August 2013. (U.S. Forest Service)

Forest fires can have a significant effect on the amount of water flowing in nearby rivers and streams, and the impact can continue even years after the smoke clears.

Now, with the number of forest fires on the rise in the western United States, they are increasingly influencing the region’s water supply, and increasing the risk for flooding and landslides, according to a study published today in the Proceedings of the National Academy of Sciences.

Researchers examined stream flow—a measure of water volume over time in rivers and streams—and climate data for 179 river basins. (Basins are areas of land where precipitation collects and drains into a common outlet.) All of the areas were located in the western United States, and all had been affected by forest fires between 1984 and 2020.

Using a mathematical model they developed, the scientists discovered that stream flow in the years after a fire tended to be higher than scientists would expect based solely on climate conditions, and that larger fires tended to be followed by larger increases in stream flow.

In basins where more than 20 percent of the forest had burned, stream flow was an average of 30 percent greater than would be expected based on climate conditions alone. The effect last on average six years.

Lead author Park Williams of the University of California, Los Angeles, said forest fires enhance stream flow because they burn away vegetation that would otherwise draw water from soil, and block precipitation before it ever reaches the soil. Intense forest fires can also “cook” soils, making them temporarily water repellent, he said. Williams is also an adjunct researcher at Columbia University’s Lamont-Doherty Earth Observatory.

From 1984 through 2020, the amount of forested area burned each year in the West increased elevenfold. That trend is expected to continue or even accelerate due to climate change.

“As a result, we’re starting to see sequences of years when large portions of forest are burned across some very important hydrological basins such as those in California’s Sierra Nevada,” Williams said.

The study suggests that wildfires will soon become yet another important consideration for those in charge of the supply and distribution of water. Each year, the region’s water managers must carefully calculate how much water will be available, and determine how to conserve and allocate it.

In one sense, the increase in stream flow from forest fires may be beneficial, Williams said. This could come as good news to dry cities like Los Angeles, because it could actually enhance water availability,” he said.

But other outcomes could be troubling. For example, in the coming decades, too much water could overwhelm reservoirs and other infrastructure, and could increase the risk for catastrophic flooding and landslides in and around burn areas.

To adapt to increasing flood risks, Williams said, water managers in California may have to lower the water levels in reservoirs in the fall and winter to make room for excess water from major rainfall and snowstorms. Such a strategy could avoid disastrous flooding in some cases, but could also put communities at risk for having too little water during the state’s increasingly hot, dry summers.

Water after a forest fire also tends to be heavily polluted, carrying mud, debris and large sediment loads. So even if the quantity of available water increases after a large fire, it’s likely that water quality will worsen, he said.

Williams said he hopes the findings help water managers and climate scientists make better predictions about water availability and flood risk.

“Water is a really heavy and destructive thing,” he said. “It’s great when it comes to us in the expected amount. It is catastrophic when it shows up unexpectedly.”

The study was coauthored by Jason Smerdon, Benjamin Cook, Arianna Varuolo-Clarke, Justin Mankin, and Caroline Juang, all of Lamont-Doherty; and researchers from the University of Colorado, Boulder; and the Cary Institute of Ecosystem Studies.

Adapted from a press release by the University of California, Los Angeles.