TWISP, Okanogan County — Three small dams in the upper Benson Creek area collapsed Thursday, contributing to flooding and mudslides that destroyed homes, took out fences and irrigation equipment, and dug huge channels south of Twisp.
The dams, known as the Wenner Lakes six miles southeast of Twisp, are private and used to hold back water for irrigation and recreation, said state Department of Ecology spokesman Dan Partridge.
Two other lakes with dams in the area used by the state Department of Fish and Wildlife did not fail, he said.
Partridge said the largest failure occurred on the Hawkins Dam, which is 35 feet high and 350 feet long, with a 4-acre lake. He said about 40 percent of the water is still in the lake, which had the capacity to hold a 40-foot-deep football field.
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“What happened, basically, is that the spillways on these dams got clogged up,” he said. A storm cell settled over the area and dumped 2 inches of rain during Thursday’s flash flooding.
Because vegetation on surrounding hillsides had been scoured by the Carlton complex fires, dead trees and debris flowed down into the lakes and prevented water from flowing out. That’s when the dams collapsed.
If the spillways hadn’t clogged, Partridge said, these lakes would have been able to handle the extra volume of water. “This was an unfortunate combination of circumstances,” he said.
The state monitors and inspects one of the three dams that collapsed. The other two are too small for state oversight, he said.
Ecology officials were on site all weekend overseeing repair work on the Hawkins Dam, and on clearing spillways of two other dams that did not breach.
Partridge said the work was done by private contractors, with the dam owners paying the cost.
Lots of other repairs from Thursday’s flash flooding continued over the weekend.
The Okanogan County Public Works Department warned of delays of up to 20 minutes on Old Highway 97 and Chiliwist Road between 6:30 a.m. and 4:30 p.m. this week.
Other county roads damaged by the storm are still being assessed, and several roads have only one lane open to traffic.