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Study: Southwest Wildfires Boost Stream Flows

Wildfires in the Lower Colorado basin can increase streamflow in the river, even during dry conditions, and even while Arizona endures a 20-plus-year drought, according to research published in this week's Nature Communications.

The 30-year study examined long-term regional fire effects at 168 sites across the contiguous U.S.

Dennis Hallema, a research fellow with the U.S. Forest Service in Raleigh, North Carolina, worked on the study. He said such links vary by region.

"For example, in Arizona we've studied four basins, and one of the basins had a 600 percent increase in flow that we could attribute to a particular fire."

Conversely, in the subtropical Southeast, no appreciable water flow changes took place following prescribed burns.

"The response of water resources to these wildfires is really dependent on local conditions," he said.

Stream flows surge most in the semi-arid Lower Colorado region and in the Pacific Northwest, and occur in areas where burns cover at least 19 percent of a river's watershed.

"If a drainage area is affected for more than a fifth of its area, then you start really seeing the wildland fire impact," said Hallema.

In Arizona, hot fires alter soil's ability to absorb water, increasing runoff.

The results can prove more problematic than beneficial. Surface flows strip the land of soil needed for regrowth. The resulting sediment load, which includes ash and toxic substances from the fire, can then choke rivers, damage aquatic ecosystems and harm downstream habitats and fisheries.

Representatives of Salt River Project, the largest provider of water to the Phoenix metropolitan area, said in an email that the result is “extremely expensive to treat and undesirable as a source [of] water.”

Nicholas Gerbis was a senior field correspondent for KJZZ from 2016 to 2024.