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NW Fishletter #375, November 6, 2017

[10] A Decade After Dam Removal, Salmonids Rebounding in Sandy River

Portland General Electric decided 10 years ago that removing Marmot Dam would be more cost-effective than updating it to meet Endangered Species Act and federal-relicensing requirements.

The 50-foot high, 200-foot-long dam on the Sandy River, a Columbia River tributary, was demolished in 2007. The structure was built in 1913, and it brought electricity to the growing city of Portland about 40 miles away. The hydroelectric project was reconstructed in 1989.

A decade after the dam was torn apart and hauled away, Oregon Department of Fish and Wildlife reports that returning wild spring Chinook numbers have increased in the Sandy from an average of 809 fish before dam removal to 2,086 fish since. Similarly, coho increased from 784 to 1,959, and wild winter steelhead increased from 898 to 2,757.

Marmot Dam had an adult fish ladder, but without the dam and its ancillary structures, such as canals, penstocks and a diversion dam, the river's salmon, steelhead and other native species now have unfettered access to the 56-mile-long river, much of it consisting of high-quality habitat for both anadromous and resident fish.

An Oregon Department of Fish and Wildlife October news release said that since dam removal, the agency has counted the largest returns in 40 years for all three species.

The increases were not solely due to dam removal. ODFW cited changes in hatchery practices as contributing to the improvements.

Marmot Dam is the largest dam to be removed in Oregon, and its decommissioning had many challenges, including the disposition of some 980,000 cubic yards of sediment built up behind the structure. After deconstruction, the dam's concrete chunks were recycled and used for road surfacing. -L. B.

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