Photo by Marcy StamperInvestigators from the U.S. Forest Service were already at work on Friday near where the Twisp River Fire appears to have started.

Photo by Marcy Stamper
A report by DNR investigators released last week determined that the Twisp River Fire was caused by tree branches rubbing against a powerline conductor on the hillside behind this house. Two days after the fire began last August, investigators had surrounded the house to search for clues.

Investigation rules out other possible causes for 2015’s fatal fire

By Marcy Stamper

An investigation into the cause of the Twisp River Fire has concluded that the August 2015 fire ignited when small tree branches, blown by wind, rubbed against a powerline conductor.

“The wildfire was determined to be caused by the branches of a water birch tree, being a capable fuel that would sustain combustion, coming in contact with and chafing an uninsulated energized power line conductor,” according to an investigation by the Washington Department of Natural Resources (DNR).

Details of the nine-month-long investigation were included in a 24-page supplement to the complete investigation. The supplement was made available to the media on Friday (May 20).

DNR investigators arrived on the scene just two hours after the fire began last Aug. 19 and focused on a V-shaped burn pattern about 6 miles west of Twisp on Twisp River Road.

That afternoon, DNR Wildland Fire Investigator John Morgan observed tree branches rubbing against the overhead powerline, but only when the branches swayed in a light wind. Powerlines in that area are part of the network operated by the Okanogan County Electric Co-operative (OCEC).

At noon on Aug. 19, about half an hour before the fire was reported, the average wind speed was 6 miles per hour, according to the report. Although the wind was relatively light, the temperature was 91 degrees with a relative humidity of 21 percent and a probability of ignition of more than 85 percent, considered very dangerous, the report said.

Even though temperatures at 4 a.m. the day after the fire started were considerably lower — in the 60s, with humidity of 46 percent — “what was notable … was the ease in which the fuels ignited and rapidly spread with intensity,” wrote the investigators.

The fire killed three young firefighters and injured four others, one severely, in an entrapment on a road just east of where the fire began. The entrapment was reported about two-and-a-half hours after the fire started.

As the Twisp River Fire intensified, it raced toward Twisp, burning 11,000 acres and destroying half-a-dozen homes, before it was controlled later that week.

In more thorough examinations over the next few days, DNR investigators found tree branches between 1/4 and 3/8 inch that had been burned at their ends, as well as evidence of charred electrical conductors. The branches were noticeably squared off and hedge-like in appearance and had rust-brown foliage, unlike green foliage on most trees in the area, according to Daniel Gregory, another DNR fire investigator quoted in the report.

The tree and powerline were located behind an small, unoccupied house on Twisp River Road. Investigators found no sign of tampering or forced entry at the house.

Based on investigators’ findings and eyewitness accounts of firefighters and area residents, the fire burned upslope from the apparent ignition point and was driven east and north by wind. The small house was not damaged and the surrounding area was not severely burned, as the fire intensified only after it spread beyond the ignition point.

Co-op maintenance

Gregory spoke with two OCEC linemen who were inspecting a power pole near the suspected ignition point the day after the fire began. Lineman Chris Zahn informed Gregory that their inspection revealed no sign of damage to the transformer, insulators or fuse link on the power pole, according to the report.

OCEC’s service handbook specifies a 15-foot corridor of cleared vegetation around powerlines, according to the investigation. OCEC Operations Manager Glenn Huber told investigators that the co-op typically clears 10 feet on each side of a powerline, creating a 20-foot corridor.

Investigators noted that the water birch in question was 29 feet from the conductor, putting it 19 feet beyond OCEC’s clearance area. However, the tree angled as it grew, which is what investigators concluded allowed its branches to come into contact with the powerline when the wind blew. The branch they took as evidence was determined to be 4 1/2 years old.

According to OCEC records cited in the investigation, vegetation in the Twisp River area was last cleared during the winter of 2012-13. The co-op conducts this maintenance on a three-year cycle, said the investigators.

David Gottula, general manager of the electric co-op, could not be reached by press time. In an interview last November, following indications that investigators believed the fire may have been caused by a powerline, Gottula said the utility had done its own investigation and provided the information to the U.S. Forest Service for its review. (Because firefighters from so many agencies were involved, both state and federal agencies conducted investigations.)

“We have given them everything we have. Our books are totally open,” said Gottula in November. The co-op is fully insured, he said.

In examining the tree branches up close two days after the fire began, investigators found a charcoal-like residue, “which was further indicative that the ends were obviously exposed to the presence of enough heat to ignite a fire,” the investigators wrote.

There was no sign of charring further down on the branches to indicate that fire had started lower on the tree, rather than from contact with the conductor, the investigators said.

The co-op records “blinks” that indicate an interruption in service, but these are provided only as daily totals, without a specific time stamp. On Aug. 19, the day the fire began, there was only one blink, which was when the co-op de-energized the line after the fire started.

Investigators found a few old cans and bottles under leaf litter and soil that suggested the presence of a garbage dump near the ignition point, but ruled them out as the cause of the fire because the garbage was corroding and appeared to have been there for some time. Investigators also ruled out lightning, campfires and smoking. They found no animal or bird carcasses that would indicate the fire had been caused by the electrocution and combustion of an animal.

DNR’s complete investigation, at 175 pages, was too large to provide digitally and has been sent to the Methow Valley News in response to a public records request, according to DNR’s public disclosure coordinator. It was not available at press time.

The supplement to the investigation does not address negligence, compensation for damages or any legal issues connected with the blaze.

The DNR is required to investigate forest fires on state and private land it protects.