Concerns rise that incoming storm may sweep toxic ash, sediment into Sonoma County waterways

A multi-day bout of heavy rainfall this weekend in the wake of the massive Kincade fire has officials and environmentalists concerned about runoff sweeping toxic ash and sediment into Sonoma County creeks and the Russian River.

An atmospheric river expected to hit the county Saturday could drop 4-6 inches of rain over the nearly 78,000 acres scorched by the Kincade fire five weeks ago, leaving behind the remains of 374 ?buildings, including 174 ?homes, now exposed to the season’s first major storm.

The short time between fire and water afforded scant opportunity for crews hired by the county to stem harmful erosion from piles of ash on hundreds of at-risk sites.

“We’re not thrilled by the prospect of runoff reaching our creeks,” said Don McEnhill, executive director of the Healdsburg-based Russian Riverkeeper, one of three nonprofits hired by the county to combat erosion.

McEnhill said his team was idled Friday by the inability to secure landowner permission to proceed with $300,000 worth of county-funded work that includes placing straw logs on burn sites to filter ash from water running off private property.

And the post-fire woes are far broader than that, said county Supervisor James Gore, whose district covers most of the Kincade fire footprint.

Once rainfall saturates the soil, runoff will occur even on land untouched by the fire and carry sediment into creeks flowing through areas like Alexander, Knights and Franz valleys and ultimately the river, he said.

Sediment clogs the gravel beds where protected salmon and steelhead trout spawn throughout the Russian River watershed and can also exacerbate flooding, Gore said.

“We have to expect we’re going to get hammered by atmospheric rivers,” he said.

A worst-case scenario, Gore said, is the type of debris flow of mud, boulders and tree branches that slammed into Montecito in Santa Barbara County in January 2018 after a deluge over the sprawling Thomas fire burn zone. Twenty-three people were killed in the mudflows.

Nothing like that is envisioned here, Gore added, but the environmental impact of the oncoming atmospheric river can’t be overlooked.

Landowners are responsible for controlling erosion on their own property, and have plenty of incentive, he said.

“They don’t want their topsoil to flow out into the Pacific Ocean,” Gore said.

As wildfires grow more frequent and expansive throughout the western United States, sedimentation becomes a greater problem, according to a U.S. Geological Survey report in 2017.

Fires burn away ground cover and vegetation, exposing bare soil more susceptible to erosion, the USGS said. Heat can also cause soil surfaces to harden and become less permeable, prompting water to rush downslope rather than soak into the ground, the report said.

In a study that covered 471 large watersheds throughout the West, USGS scientists determined that by 2050 the amount of sediment could at least double in more than one-third of the watersheds.

Increased sediment may impact water supply by reducing reservoir storage, the report said.

At Pepperwood Preserve, a 3,200-acre tract northeast of Santa Rosa dedicated to conservation science, preserve manager Michael Gillogly was out Friday, by himself, digging water diversion bars to limit erosion on roads flattened by firefighting bulldozers during the Kincade fire.

About half of the preserve was charred by both the 2017 Tubbs and Kincade fires.

“When I read what’s coming I thought I better get out here,” Gillogly said.

You can reach Staff Writer Guy Kovner at 707-521-5457 or guy.kovner@pressdemocrat.com. On Twitter @guykovner.