What Started the Biggest Fire in California History? Yellowjackets, and a Man With a Hammer

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Teenage resident of Clearlake Oaks fought to save his home as the Ranch Fire advanced through the area on Aug. 4, 2018, a week after it started on a ranch near the Mendocino County community of Potter Valley.  (Noah Berger/AFP-Getty Images)

What did it take to start the biggest wildland fire in California history? A rancher attempting a simple chore, a nest of angry yellowjackets and some very bad luck.

Cal Fire reported Thursday that its investigation of the Ranch Fire, which started last July 27 near Clear Lake and eventually burned a sprawling expanse of forest and grassland 13 times the size of San Francisco, was touched off by an unidentified man trying to hammer a metal stake into the ground.

Wildfires in California

The agency's report says investigators determined that the hammering threw off sparks or hot fragments that ignited a small patch of dry grass that was 2 to 3 feet tall. The blaze surged uphill despite the panicked efforts of the man who told arriving investigators he had started it.

The man called firefighters to his property just off Highway 20, northeast of Clear Lake and south of the Mendocino County community of Potter Valley, and told them the fire began with what sounded like a straightforward ranch job.

The previous winter, a 50- to 60-foot length of fabric that was suspended over several water tanks as a sunshade blew down in a storm. The rancher's daughter had complained last July that water in the tanks was too hot for livestock to drink, the man told investigators. So late on the morning of July 27, he drove up the hill from his home with tools and supplies to reinstall the sunshade.

The rancher told Cal Fire that when he picked up the fabric, he disturbed a yellowjackets' nest underground and was confronted with a swarm of the stinging insects. Since he's allergic to bees, he said, he backed off for about an hour to let the yellowjackets calm down.

When he returned, he brought a claw hammer and a metal stake that he intended to use to plug the small hole leading to the yellowjackets' nest.


The man told investigators that "as quickly as he could," he drove the stake about a foot into the ground, blocking the hole.

But "while hammering the stake, he smelled smoke," the Cal Fire report says. "He said he looked down, directly behind him, and saw the vegetation fire he described as 2 feet by 2 feet in size."

The rancher said he made a series of frantic attempts to stop the fire from spreading.

He said he grabbed a shovel to try to extinguish the flames, but the ground was so hard he couldn't scrape up enough dirt to have an effect. He then tried to smother the fire with a rug and trampoline lying nearby, but the trampoline caught fire.

Next, he grabbed a plastic hose next to the water tanks. But heat from the fire had already caused a kink in the line, and he lost water pressure.

He also tried to break a water line from the tanks and direct the resulting flow at the escaping blaze, but again didn't have enough water pressure to stop the flames.

Finally, he said, he got in his four-wheeled utility vehicle and tried to drive ahead of the fire and "kick up dirt" to stop it. That effort came to an end when he lost control of the vehicle and crashed.

He told investigators that at that point he ran to his residence and phoned Cal Fire to respond to the blaze, which he believed covered about an acre as it spread up the hillside from the water tanks.

Cal Fire investigators recovered the hammer, stake and metal fragments from the site of the fire's ignition. The agency's report says all other possible causes for the fire were eliminated.

The Ranch Fire took nearly two months to contain and eventually burned 410,203 acres -- 640 square miles. The Mendocino Complex Fire, which included the Ranch blaze and the nearby River Fire -- burned a total of 459,123 acres and ranks as the largest fire by land area in state history.

One firefighter died battling the blaze —- killed by a tree snapped by a low-altitude fire retardant drop. Three other firefighters were injured. In all, the Mendocino Complex destroyed a total of 280 structures, including 157 residences.

The total cost of fighting the two fires has been estimated at more than $200 million.