Photo by Ann McCreary Standing on the west side of the Twisp River where it curves past the town park, Andrew Denham, Twisp public works supervisor, points toward ice chunks that remained along the river banks this week after a large mass of ice temporarily dammed the river near the Highway 20 Bridge.

Photo by Ann McCreary
Standing on the west side of the Twisp River where it curves past the town park, Andrew Denham, Twisp public works supervisor, points toward ice chunks that remained along the river banks this week after a large mass of ice temporarily dammed the river near the Highway 20 Bridge.

Ice, logs, debris blocked river
after sub-zero temperatures

By Ann McCreary and Marcy Stamper

Two large ice dams that blocked sections of the Twisp River last week created concerns about potential flooding in Twisp, particularly at the River’s Edge Mobile Home Park, until the dams broke open on their own over the weekend.

The ice developed during several days of sub-freezing temperatures in late December, and by Wednesday morning (Jan. 6) immense chunks of ice, carrying large logs and other debris, had traveled several miles down the river and choked off the flow of water near the Twisp River Bridge at Highway 20, and upriver near the Twisp River Ponds just west of town.

Although the river was no longer blocked early this week, huge chunks of ice were piled up to 10 feet high along the river banks near the ponds, and Twisp officials were keeping a wary eye on the situation.

Andrew Denham, the Twisp Public Works supervisor, was alerted to the ice buildup near the bridge by Police Chief Paul Budrow early in the morning on Wednesday, Jan. 6.

The ice and debris blocked a section of river about 1,500 feet long, extending from upstream of the Twisp River Bridge to the town park, Denham said.

Photo by Marcy Stamper The Twisp River had backed up to the point that excess water flowed into a side channel.

Photo by Marcy Stamper

The Twisp River had backed up to the point that excess water flowed into a side channel.

“About half of the ice got stuck in the main channel above the ponds, and about half continued through and went under the bridge on State Route 20 and stopped about where the park is,” Denham said.

“There was a mass of stationary ice large enough to back the water up … all the way to the Horseshoe Trailer Park [a.k.a. River’s Edge],” he said.

The water level near the bridge rose about 9 feet at its peak on Jan. 6 and came within 4 feet of the top of the levee by the town park, Denham said.

Alarmed by noise

Denham said he talked with residents along the Twisp River who said they had been alarmed by the loud noise of the ice as it broke trees and dragged debris along with it as it surged down the river last week.

“We were on edge for a little bit. My mom has been checking — it was right behind her house” in the mobile home park, said Zitella Gates on Tuesday (Jan. 12). “The water has gone down quite a bit now.”

Jennifer Molesworth, the Methow sub-basin liaison for the U.S. Bureau of Reclamation, was heading up Twisp River Road on Jan. 6 when she noticed that “something had sculpted the banks of the river.” Even over the weekend, after the weather warmed and the ice dam had started to break up, snow on the banks of the river appeared sheared off at a sharp angle, rather than piled in rounded mounds.

When Denham first saw the dam at the Highway 20 Bridge, “I thought this was the only ice dam. Then I went upriver and saw the other ice dam, which was equally as big,” he said.

He was alarmed by the possibility of the upper dam breaking open and dumping water and ice downstream while the lower dam was still lodged in place. “It would have been impossible to prevent flooding,” Denham said.

“The water and ice continued to rise [during the morning of Jan. 6] because it was blocked all the way up to the trailer-park area, until it created enough pressure to create a relief hole along the levee by the park,” Denham said.

By noon “the water was being relieved out of a side channel, but not the main channel,” he said.

Team assessment

Denham called a meeting on Friday (Jan. 8) to discuss the ice dam with representatives of several agencies, including the U.S. Army Corps of Engineers, the Yakama Nations, Methow Salmon Recovery Foundation (MSRF, which owns the Twisp Ponds property), the Bureau of Reclamation, Washington Department of Fish and Wildlife (WDFW) and Okanogan County emergency management.

IceDamQuote“We took them to tour both ice dams. We decided our best approach was to watch and monitor it in hopes it would self-mitigate,” Denham said. “We had the perfect weather for it to self-mitigate, which was the deciding factor. If it was not conducive I had gotten approval of everyone at the table and would have gotten in the river and broken up the jam [near the bridge].”

The group assessed the likelihood of ice backing up and threatening the mobile home park, Molesworth said. “We looked around and saw how it had changed from Wednesday to Friday. It was melting and the weather looked good.”

“I appreciated how Andrew got everyone together — people who knew about the river and could do something about it,” she said.

The meeting was helpful because it built relationships and a system for addressing an emergency, said Okanogan County Manager Maurice Goodall.

While the river appeared to be covered with a thick layer of ice last Wednesday and Thursday, particularly downstream of the bridge, the fact that there was no water pooling near the ponds indicated that the river was still flowing beneath the ice, said Chris Johnson, president of MSRF.

Although the channel was very narrow last week — just 1 or 2 feet wide — it has since widened to 20 feet, which is quite normal for winter, Johnson said. “There was never a complete blockage of the river,” he said.

MSRF commissioned two drone flights on Friday and Saturday (Jan. 8 and 9) to get an aerial view. The videos show the river almost completely covered by ice and debris near the town park. “You can see why Andrew was concerned,” Johnson said.

“The drone showed strong fissures in the ice with the water still flowing,” said Johnson.

The side channels, particularly near the ponds, functioned as a relief valve. “Wherever the river can spread out, it unloads and gets out of the channel,” Molesworth said.

Contingency plans

Denham decided to apply for a permit from WDFW that would allow an excavator to enter the river to break up the ice if needed to prevent flooding.

Called a hydraulic-project approval, the permit is good through Jan. 20, Denham said. A contract was quickly drawn up with a local contractor for the excavator work if necessary.

Denham said plans called for bringing the excavator into the Twisp River across from the park, just above the confluence with the Methow River.

Because Twisp River makes an “S” curve as it flows through town and under the Highway 20 bridge, it creates a bottleneck where ice has been caught in the past. The levee, built decades ago to protect the park and city property, exacerbates the situation by forcing water and ice into a confined channel.

“There are major concerns about these bottlenecks in the Twisp River,” Denham said.

Representatives of the Army Corps, WDFW and MSRF agreed that funding should be sought to modify the levee or take other steps to reduce the risk of future ice dams and flooding, Denham said.

Any changes contemplated would not alter the levee near the mobile home park, which did what it was supposed to do to protect the homes there, said Johnson.

This was at least the third “ice event” since they acquired the Twisp Ponds property in 2002, said Johnson. Removing levees on both sides of the river gives the ice somewhere to go, relieving the pressure on the town, park and highway bridge, he said.

This is what is called a “bank-full event,” as opposed to a true flood event that could be seen during spring runoff, said Molesworth.

As the extremely cold weather moderated, the buildup of ice slowed and the threat is diminishing this week, Denham said Tuesday (Jan. 12). Large chunks of ice are continuing to break off and float downriver, he said.

“There’s no concern anymore in our mind at this point, although with another week of sub-zero temperatures, it could easily repeat itself,” said Johnson.