Over the past decade, both the number and size of dams removed on rivers across the United States has been increasing and those removals typically involves release of at least some of the sediment stored in the reservoir behind the former dam.
As released sediment moves downstream, it has the potential to dramatically change the form and behavior of the downstream channel.
Nowhere has this been more closely studied than on the Sandy River, outside Portland, Oreg., following the removal of Marmot Dam in 2007. At the time, its removal produced the largest intentional release of sediment from any dam removal in history.
A newly published U.S. Geological Survey report describes how the Sandy River responded to the release of sediment over the next two years. It documents the rapid and dramatic changes in channel form, profile, and sediment transport close to the dam site.
For more than 90 years, Marmot Dam blocked the Sandy River, providing hydroelectric power by diverting water into Roslyn Lake where generations of Portlanders fished and swam. But a combination of economic and environmental issues resulted in the dam’s owner, Portland General Electric, surrendering its operating license and removing the dam. Similar issues are prompting dam removals across the country.
“An important management issue associated with dam removal is the fate of sediment accumulated in reservoir pools,” explained Jon Major, the report’s lead author and a hydrologist with the USGS. “Concerns over dam removal are sharpened where stored sediment may be contaminated by decades of upstream land-use actions.”
“Removal of Marmot Dam provided the research community with an exceptional laboratory for studying how powerful rivers digest large quantities of coarse sediment,” says coauthor Gordon Grant, a hydrologist with the U.S. Forest Service. “Our study of physical response of the river system to the dam removal confirmed some pre-removal predictions, but also revealed some surprises, and the lessons learned have broad implications for helping to guide future removals.”
-- An energetic river can rapidly incise and remove large volumes of unconsolidated stored sediment, even under very modest flows.
-- Channel change is initially quite rapid but diminishes over time as sediment sources diminish.
-- Allowing rivers to naturally process stored sediment rather than manually removing it before dam removal may be a tractable option for coarse, clean sediment in cases where sediment deposition will not create a flood risk downstream.
The report, “Geomorphic Response of the Sandy River, Oregon, to Removal of Marmot Dam,” was recently published by the USGS. It is coauthored by Jon Major and Jim O’Connor (USGS), Charles Podolak (Johns Hopkins University), Mackenzie Keith (USGS), Gordon Grant (USDA Forest Service), and others.
Read the report online at http://pubs.usgs.gov/pp/1792/pp1792_text.pdf
For photos go to http://www.fs.fed.us/pnw/news/2012/06/marmot-dam.shtml