Oct. 19, 2007

Coffer dam breach frees Sandy River
Threatened fish benefit as PGE completes dam removal

Sandy, Ore. — Mother Nature triggered the last phase of the Marmot Dam removal on the Sandy River today. As anticipated, the first high- river flows of the season breached the earthen coffer dam that held back water during Portland General Electric’s demolition activities at the site.

Crews finished removing the 47-foot-high Marmot Dam on September 30 after a series of blasts and excavations that began July 24. Before that started, workers built an eight- to nine-foot-high coffer dam on top of the reservoir sediment to keep the work area dry. (The coffer dam forced the Sandy River to bypass the old dam site.) Scientists and regulators had decided the best thing for the river would be to allow its autumn flows to remove the coffer dam and distribute accumulated sediment behind the dam. This mimics the natural events that give the SandyRiverits name. This aspect of dam removal will be complete when the last pieces of the coffer dam and reservoir sediment wash downstream.

In 1999, PGE decided to remove Marmot Dam and the rest of its Bull Run Hydroelectric Project. The utility was committed to improving conditions for threatened salmon and steelhead. Its analysis showed that removal was less expensive to customers than upgrading the facility with new salmon and steelhead protection measures and years of expensive maintenance.

As the Sandy River rose, demolition crews helped the coffer dam breaching along. They removed sump pumps in the coffer dam, making it wetter and more likely to slide downstream. An excavator accelerated flow by cutting a 25-foot-wide notch on the south side of the coffer dam. Then, earth movers dammed the bypass channel that had routed river water around the construction site, building additional water pressure on the coffer dam.

During that process, the temporary fish passage system was removed. It had captured adult migrating salmon for sorting and transport above the dam.

With the coffer dam gone, salmon and steelhead will now be able to navigate upstream without a fish ladder or human help for the first time since Marmot Dam construction in the early 20th Century.

For additional details, photos, videos and background on the dam removal, visit www.MarmotDam.com.

Late next spring or early next summer, PGE will remove the project’s other dam, the Little Sandy Dam, on its namesake river. That process will include draining RoslynLakeand restoring its bed to natural contours and vegetation. The other steps in the hydro project removal will be complete by the end of 2009.

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