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while using only a fraction of the Sandy River's normal water flow. This required construction of a series of canals and tunnels to combine the water of the Sandy with that of the Little Sandy River and get it to the powerhouse, completed in 1912.

Since that time, Marmot Dam has enjoyed regular upgrades and enhancements. Many, like the fish screens added in 1951, were designed to improve the migration success of salmon and steelhead. In 1989, the original timber structure was replaced with concrete.