That it hasn’t rained much this year isn’t all bad news, especially in the aftermath of the Creek Fire that burned nearly 40% of the San Joaquin River watershed.

Most importantly, mountain communities devastated by the Creek Fire have not faced the secondary disaster that can be brought by weather, like in Santa Barbara County when heavy rain in the burn scar of the Thomas Fire led to deadly and destructive mudslides. Some areas near Big Creek and North Fork are at risk of hazardous, post-fire debris flows.

There is another benefit of the dry year. A big rainstorm could make tap water undrinkable for thousands of people in Fresno County, where public water systems aren’t equipped to filter large amounts of sediment that could come from debris flows.

No storm of that magnitude is expected at this time, but regional water officials are “definitely watching it,” and checking rain forecasts daily, according to Tricia Wathen, an engineer who heads the Central California office of the state Division of Drinking Water.

Some water systems in the southern Sierra Nevada were directly and immediately damaged by the fires last fall: The SQF Complex fire destroyed water system infrastructure at both Alpine Village and Sequoia Crest, causing drinking water to be temporarily at risk, and a boil-water notice was briefly in place.

In the community of Big Creek, more than 40 homes were destroyed, along with basic equipment like water meters and hoses.

Water quality issues run downstream from fire

Wildfires can have devastating effects on drinking water access for communities that rely on the watershed for their running water, both because of potential contamination from chemicals used during firefighting, and because of the increased potential for debris flow, flooding and mudslides.

In the case of the Creek Fire, that includes cities and communities that rely on surface water from the San Joaquin River watershed, including the cities of Clovis, Fresno and Orange Cove, and smaller systems in Brighton Crest, Friant, Hidden Estates, Tesoro Viejo and Big Creek.

Wathen said she is not aware of any big concern for contamination from fire retardants or firefighting foam. The foam used in the fire is supposed to be free of PFAS, a category of chemicals with known health risks. Community water systems are testing their water for common contaminants.

“There is obviously a concern of the burn scar and what could be happening with debris flow after a high rain event,” Wathen said.

That’s because a fire causes a loss of vegetation, and exposed soil can result in a flow of mud and debris into creeks and streams. In a natural river system, the debris would flow down into the river, but the San Joaquin River flow is interrupted by several dams, so debris and sediment will likely be trapped in the reservoirs. The fire burned nearly 380,000 acres in the western slopes of the southern Sierra Nevada.

Communities who receive surface water from those reservoirs pull water from intakes that are far below the surface of the lake; sediment buildup and turbidity, or particles, can make the water unsafe to drink if not properly treated.

Water systems operated by Fresno and Clovis would likely be able to handle the turbidity (cloudiness from particles) from a debris flow event, or they would be able to turn off the valves that deliver surface water and rely wholly on groundwater, Wathen said.

That’s not an option for smaller rural communities that don’t have access to groundwater supplies, or have older water treatment plants. Many of those communities already have contaminated water. In those cases, the communities would be issued a notice that their water is unsafe and they would need to boil it before use.

“Orange Cove is an older plant that can’t manage fluctuations,” Wathen said, offering an example. “If it was like mud or sludge and clog up the filters, that’s a big concern.”

It’s unclear how long it would take after a rain event for the drinking water to be affected. Wathen said modeling isn’t available to provide those estimates. For now, communities are monitoring daily for turbidity and other constitutions, she said.

“And if we start to see increases, or if we see there is a high rain event, that will be significant. If they are expecting an inch in 30 minutes, that is significant,” Wathen said.

The threat of an atmospheric river

Mountain communities were threatened by a potential disaster in January, when an atmospheric river dumped a lot of rain in a short time frame. Heavy rain could melt snow and carry massive amounts of debris downstream into reservoirs.

Luckily, however, that rain event was cold and dropped snow instead of rain, according to Jim Dudley, meteorologist and lead forecaster with the National Weather Service, Hanford office.

Looking forward, he said, there is a chance of light precipitation Saturday, with rain and some snow at higher elevations in the Sierra mountains and San Joaquin drainage, but not enough to affect water quality or turbidity.

Any forecasting beyond seven to ten days is “very, very difficult,” he said.

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