Sandy River flows free

Rain brings enough water to wash away coffer dam

By Garth Guibord

The Sandy Post, Oct 24, 2007, Updated Oct 30, 2009

Peter Wylie / The Sandy Post

A forklift scratches a notch in the top of the coffer dam to start the flow of water that will remove the dam.

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For the first time in approximately 100 years, salmon have free rein of the Sandy River, thanks to the influx of water that breached an earthen coffer dam – the last phase of the removal of the Marmot Dam on the Sandy River on Friday, Oct. 19.

“It was pretty amazing,” said John Esler, project manager for Portland General Electric (PGE), which decommissioned the dam almost a century after it was built. “It was an incredible force of nature as the river sought a balance.”

Demolition of the Marmot Dam began July 24, when workers built the approximately 9-foot-tall coffer dam behind the Marmot Dam to re-direct the Sandy River around it. The project is part of PGE’s decommissioning of the Bull Run hydroelectric plant, which also will include the demolition of the Little Sandy Dam on the Little Sandy River next spring.

Crews finished the demolition of the Marmot Dam on Sunday, Sept. 30, then waited for the first high water event of the fall to remove the coffer dam and much of the approximately 750,000 cubic yards of sediment that had built up behind the Marmot Dam over the course of its life.

Friday offered that opportunity, and workers closed off the side channel of diverted water and made a notch in the coffer dam for water to pour over.

“It went real slow for 15 minutes,” Esler said. “It kept working, and with the water rising, it finally cut enough of a channel and it unzipped. Then it just roared as the material just went downstream.”

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