May 11, 2008
Photo: Dwight Tanner, USGS
The number of large and small dams being removed from U.S. rivers is small but increasing, as both river restoration gains popularity and aging dams lose their license from the U.S. Federal Energy Regulatory Commission, according to the March 2008 issue of Geotimes.
Dam removals, however, bring ecological and geological unknowns.
Geotimes covers the story of Marmot Dam in northwestern Oregon to learn more about what happens when a human-made structure is removed from a river after 94 years. Water flow, sedimentation fluctuations and ecological changes could occur but little is known about the natural processes involved when such a large structure is removed after a long period of time.
By using the Marmot Dam as a case study, future dams can be removed more easily and efficiently, according to the magazine.
Marmot Dam, Little Sandy Dam removals
Marmot Dam on the Sandy River was decommissioned by Portland General Electric, according to the U.S. Geological Survey. Little Sandy Dam, on the Little Sandy River, will be removed this summer.
The reservoir behind Marmot Dam contained approximately 900,000 cubic yards of sediment in a wedge extending upstream from the 47-foot concrete dam, according to the USGS. Dam removal has resulted in the suspension and transport of this sediment, and the near-term effects on habitat are difficult to predict.
In the long term, the streambed will return to its natural state, and summertime stream temperatures below the dam site are expected to decrease.
The Sandy and Little Sandy Rivers provide access and habitat for anadromous fish and other aquatic organisms, so there is a need to document the impacts of sediment released from these dams.
The USGS, Sandy River Basin Monitoring and Research Team, the Oregon Department of Environmental Quality and the Oregon Water Resources Department are studying these effects.
River restoration study
The groups collected data on suspended sediment concentrations, stream temperature and turbidity prior to the removal of Marmot Dam. These efforts will continue.
The future scope of the study will include the effects of removing both dams. The study’s long term objectives are to:
- Determine hourly and daily concentrations and loads of suspended sediment, both before and after dam removal, from sites above and below Marmot Dam.
- Following dam removal, determine the relative contribution of suspended sediment loads to the lower Sandy River (below Bull Run River) from potential sources including: the Sandy River above Marmot Dam, sediment stored behind Marmot Dam, and the Little Sandy River.
The study’s approach is to:
- Collect hourly discharge and turbidity data and summer water temperature.
- Collect and analyze water samples for suspended sediment concentration and fraction of fine materials over a range of flow and turbidity conditions.
The study, in future years, may:
- Develop statistical relations between instantaneous turbidity and suspended sediment for each station.
- Calculate instantaneous loads of suspended sediment using hourly turbidity data and derived turbidity-sediment regressions.
- Provide this data as input to a larger geomorphological study of dam removal.
Source: American Geological Institute, U.S. Geological Survey.