Photo by Mark Linton | A hydrologist checks for organic matter and hydrophobicity in burned soil at at the sit of the Rough fire in 2015

How wildfire can cause flooding

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by August Linton

After the consuming heat and flame of a wildfire, flooding seems an unlikely problem to have to worry about. But catastrophic floods after a section of land is burned often threaten both human settlements and ecosystems. 

Post-wildfire flooding can be caused by several processes. According to Cal Poly Humboldt professor of forestry, fire and rangeland management Dr. Jeff Kane, the forest floor normally acts as a sponge, soaking up and slowly releasing rainfall. When a wildfire burns that layer, there’s nothing left behind to stop rainfall from rushing directly downstream.

Wildfire also can release and vaporize waxy compounds from conifer needles, which then accumulate on the burned ground and form an even more water-resistant layer. 

“Instead of percolating, [water] may flow overland,” Kane said. “If you don’t have the vegetation to absorb the precipitation, then it’s going to be more impactful.”

Hydrology and watershed management professor Dr. Andrew Stubblefield says that the impacts of this flooding extend to the whole ecosystem. When the forest floor loses its ability to absorb water, the topsoil can become saturated and eventually sloughs downstream, taking with it the nutrients it stores.

“Now you have a forest that’s less able to hold water to grow trees and provide nutrients to grow trees,” said Stubblefield. “It’s impoverished, or it’s depleted; and it can take a while to hundreds of years even to rebuild the nutrients.”

This can impact what plants regrow while the land recovers from the fire and flood. Weedy, often-invasive plants may have an easier time reestablishing themselves in this less-nurturing environment than native species, according to Kane. 

Soil and nutrients washed into rivers and streams also negatively affect their ecosystems. Sediment and debris carried by flooding associated with the McKinney fire caused fish kill in the Klamath River earlier this August, as clays in the soil interrupted oxygen flow and nutrients nourished a deadly algal bloom. 

Post-wildfire flooding can also be dangerous to human settlements and to the ecosystem around it because it picks up debris and soil and carries it downstream. Debris flows caused by heavy rain after the Thomas fire in Montecito, California killed 23, injured at least 167, and damaged 408 homes, according to a 2019 research article by J.W. Kean et al. 

It’s the charge of the National Forest Service’s BAER (Burned Area Emergency Response) teams to analyze the risks for flooding after each fire and implement measures to mitigate damage and environmental impacts. According to the McKinney Post-Fire BAER incident overview, these multidisciplinary teams of scientists decide what, if anything, needs to be done to protect natural resources or human settlements in the area. 

Examples of possible interventions include seeding the hillside from the air, setting up log breaks along hills or in creek beds, or stabilizing at-risk areas with hydro-mulch (a material similar to paper mâché which also contains seeds,) said Stubblefield. Sometimes the best treatment is to warn people away from at-risk areas with weather alerts when a big storm is coming in.

But with modern fires often burning millions of acres at once, what scientists can do to mitigate flooding becomes a problem of scale. 

“The August Complex [fire] last summer was a million acres, what would it cost to try and stabilize that landscape, you know, it’s too big,” said Stubblefield. “It would be the gross domestic product of a small country.”

Flooding is not an inevitable follow-up to wildfire. It takes both intense fire and intense rain to create the right conditions. If ground cover vegetation like grasses have time to return before the next intense rain, it stabilizes the soil and can even begin to break down the waxy hydrophobic layer.

However, due to the effects of anthropogenic climate change and poor forest management, the perfect storm happens more often. 

When this land was stewarded by Native Americans, regular fires were part of that management. In the post-colonial absence of that management, white settlers saw wildfires as something that needed to be suppressed immediately, rather than allowed to run their course as a physiological process of the forest. When the dense, thick, choked forests that this policy produces catch fire, they burn hotter, longer, and over a wider area. The environmental legacy of clear-cut logging and fire-prevention has left the forests in a vulnerable and volatile state.

“We are moving into an era of active fire management,” said Stubblefield.

This means more prescribed burns, and an attitude towards fire that acknowledges its essential role in forest health. Post-fire flooding, too, is a physiological component of the ecosystem. According to Stubblefield, if sediments weren’t carried into the river, salmon might not have gravel to spawn on at all.

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