Marmot coffer dam breach. USFS scientist Gordon Grant points to rapidly
eroding knickpoint following the breach. Credit: USDA-FS Jon Major.
Ore. June 14, 2012. Over the past decade, both the number and size
of dams removed on rivers across the United States has been increasing.
Dam removal 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, Oregon, 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 (USGS) report describes how the Sandy River
responded to the release of sediment over the
next 2 years. It documents the rapid and dramatic changes in channel
form, profile, and sediment transport close to the dam site.
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
• 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.
USGS scientist Jon Major readies a time-lapse camera to capture
the breaching of Marmot Dam. Credit: USGS scientists/interns.
USGS scientists and interns from the National Center for Earth
Surface Dynamics process gravel samples to measure grain size
distributions. Credit: USGS.
The PNW Research Station is headquartered in Portland, Oregon.
It has 11 laboratories and centers located in Alaska, Oregon,
and Washington and about 425 employees.