Marmot Dam Removal


Removal of dams from the Sandy River basin reversed a historic trajectory for the basin that began when Portland General Electric (PGE) completed the Bull Run Hydropower complex in 1913. The 47-foot high Marmot dam diverted flow from the Sandy River, 30 miles from its mouth on the Columbia River. A tunnel excavated through a mountain (historically called the Devil’s Backbone because of the difficulty immigrants faced hauling wagons and gear over it on the Oregon Trail) carried diverted water into the Little Sandy River, a tributary of the Bull Run. The Little Sandy dam completely dewatered the stream, sending it along miles of flume on a train-like wooden trestle to a reservoir, then into a powerhouse along the lower Bull Run. PGE built the dams to bring electricity to the growing city of Portland 20 miles east, also powering a trolley line that brought city dwellers for boating and swimming at the reservoir, called
Roslyn Lake. The dam generated about 22 MW of electricity, enough to power about 16500 modern homes. In the late 1980s a concrete layer was added to the original earthen structure to strengthen the Marmot dam.

Faced with a requirement to comply with the ESA and its prohibition to harm listed species, PGE
weighed its options to renew its federal license. Continued operation of the dam would require constructing and operating fish passage. They would need both a ladder for spawning adults to safely return upstream, and a means to safely move migrating juveniles downstream across the dam. Recognizing that dam removal would cost customers less in the long run, PGE committed in 1999 to voluntarily removing Marmot and Little Sandy dams, and their associated infrastructure.

Planning for the physical dam removal took several years. A coalition of agencies and conservation
groups consulted with the utility on the terms and process under which PGE would withdraw its hydropower license. No such surrender of a license had occurred before, so the extent of responsibility for river conditions once the dam removed had to be defined. PGE eventually agreed to dismantle the dams, transfer related water rights to the state as instream rights, donate much of the project’s land to public ownership, prevent invasives from colonizing those lands, and monitoring river conditions for several years.

Deconstruction at Marmot began in summer 2007, with the river diverted by a temporary earthen coffer dam built behind the once permanent Marmot. On October 19, 2007, as calculated in dam removal plans, a seasonal storm rose with enough force to wash away the temporary dam. An excavator cut a small notch in the coffer dam, releasing first a trickle and then a roaring torrent. Approximately 19 hours later, the entire structure was washed away. The Sandy flowed free for the first time in 94 years.