Sourdough Fire consumes another 1,000 acres, cuts dam power to Seattle

The Sourdough Fire in the North Cascades grew more than 1,000 acres over the weekend.

The growing fire forced Seattle City Light to disconnect the Ross and Diablo dams from the power grid that serves customers in the Seattle area Saturday night. A third Skagit River power source 4 miles downstream, the 99-year-old Gorge Dam, continues to provide hydropower to Seattle.

The fire, started by lightning in the mountains above Diablo Dam on July 29, had previously led the utility to take Ross and Diablo dams offline for about a week each on Aug. 2.

Air temperatures at Diablo Dam hit 104 degrees Sunday afternoon and 100 degrees Monday afternoon. The National Weather Service issued an extreme heat warning and a “red flag warning” for extreme fire danger for the North and Central Cascades through Tuesday.

Seattle City Light officials say they’ve been acquiring power from other sources, including the utility’s own Boundary Dam in Washington’s northeast corner and buying power from other western power providers, to fill the gap, and customers shouldn’t notice anything different.

“There will be no impact in the city of Seattle whatsoever,” Seattle City Light spokesperson Jenn Strang said. “What we actually do is increase our market spending.”

“The grid has many generation sources in reserve, so it shouldn’t be a problem,” grid security consultant Ross Johnson said by email. “I would be very surprised if this fire had a material impact on Seattle’s power supply.”

The Sourdough Fire accelerated its growth as a wave of dry, hot air spread across the region over the weekend.

The North Cascades Highway has been closed since Thursday.

“Rocks continue to fall down, trees continue to fall down, so we’re not putting crews in there unless we absolutely have to for transport back and forth,” said Northwest Interagency Incident Management Team section chief Dean Lange.

Lange said the North Cascades Environmental Learning Center, Ross Dam Resort, and Seattle City Light transmission lines were in good shape, with vegetation cleared and water lines set up near vulnerable facilities.

Lange is one of more than 400 people assembled to fight the Sourdough Fire. Hundreds are living in tents in the Seattle City Light company town of Newhalem. Some crews have been camping at North Cascades National Park’s Colonial Creek Campground to avoid the rolling debris that has been landing on the stretch of Highway 20 between the firefighting zones and the Newhalem base camp.

Flames did briefly jump south of Highway 20 in one spot, but firefighters have kept them from spreading any farther south.

Helicopters have been dropping water in some areas to slow the flames’ advance. In other areas, a drone has been dropping incendiary devices known as “Dragon Eggs” to ignite vegetation between the edge of the wildfire and a control line established by firefighting crews. The controlled burns use up vegetative fuel so the burned-over area serves as a kind of fireproof barrier.

The U.S. Forest Service has incorporated drones as a safe way to both monitor and start fires from a distance in difficult terrain like the jagged North Cascades.

Dragon Eggs are brightly colored plastic spheres, smaller than a ping pong ball, that a helicopter or drone can scatter onto a forest from above. Thirty seconds after a type of alcohol is injected inside each plastic ball, a chemical reaction erupts, and the tiny ball burns like a flare for a minute or more.

The Sourdough Fire had burned nearly 3,000 acres as of Monday morning, and continued hot, dry air this week boosts its chances of spreading rapidly.

In the three months before lightning sparked the Sourdough Fire on July 29, Diablo Dam had half its normal rainfall, according to Washington state climatologist Nick Bond.

The Sourdough Fire is also the source of the haze that has spread to much of the Puget Sound region since Sunday. Late Monday afternoon, air quality just inland from Puget Sound, from Monroe to Bellevue to Puyallup, was moderate, while air closer to the shores of the sound remained good.

2:00 p.m., 8/15/2023: This story has been updated to include details on Dragon Eggs.