Dam Removal and Sediment Transport in the Sandy River Basin, Oregon

Project Chief: Dwight Tanner

Scientific Staff from the Oregon Water Science Center and the Cascades Volcano Observatory

In cooperation with the Sandy River Basin Monitoring and Research Team, funded by Portland General Electric, the Oregon Department of Environmental Quality, and the Oregon Water Resources Department

Marmot Dam on the Sandy River, before removal (photograph by Dwight Tanner, USGS)

BACKGROUND:

Marmot Dam on the Sandy River has been decommissioned by PGE and removal has been completed. (See map and schematic diagram of the Sandy River Basin.)  Little Sandy Dam, on the Little Sandy River, will be removed in the summer of 2008. The reservoir behind Marmot Dam contained approximately 900,000 cubic yards of sediment in a wedge extending upstream from the 47-foot concrete dam. 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 is cooperating with the Sandy River Basin Monitoring and Research Team, the Oregon Department of Environmental Quality, and the Oregon Water Resources Department to study these effects.

View time-lapse photos of Marmot Dam removal (requires Windows Media Player)

Listen to a podcast discussion of the dam's removal and its effects by USGS geomorphologists.

OBJECTIVES:

The study objective is to collect data on suspended sediment concentrations, stream temperature, and turbidity prior to and after removal of Marmot Dam.

The future scope of the study will include the effects of removing both dams. The study's long term objectives are to:

1. Determine hourly and daily concentrations and loads of suspended sediment, both before and after dam removal, from sites above and below Marmot Dam, and

2. 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.

APPROACH:

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 broad range of flow and turbidity conditions.

Future years of the study may include:

• 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.

DATA:

Click on the station number to obtain real-time streamflow and water quality data. Click on the list of parameters under "Custom graphs and tables" to obtaindata for user-specified periods.

Sediment and turbidity data (Excel), November 2005 through July 2006 (provisional)

Stream temperature data (Excel), June 2007 (provisional)

Water-quality cross sections (Excel), July and August 2007 (provisional)

Sandy River sediment-size analysis and concentrations, water year 2008, and sample inventory/description

Graph (PDF) showing water year 2008 flow (Q_w, in cubic meters per second), suspended-sediment load measurements (Q_ss, in kilograms per second) from transect samples and from a pump sampler at the Sandy River below Marmot Dam station, and bedload measurements (Q_b, in kilograms per second). (provisional)

Photos:

• Sampling suspended sediment from cableway at Sandy River below Marmot Dam (November 2, 2005)
• Measuring turbidity at Sandy River near Marmot (above the dam) (November 2, 2005)
• Sampling suspended sediment from cableway at Sandy River below Bull Run River (November 2, 2005)
• Marmot and Little Sandy Dam photos from Portland General Electric
• Marmot Dam, before the removal process began (March 18, 2007)
• Marmot Dam, after most of the concrete had been removed (August 9, 2007)
• Time-lapse photos of Marmot Dam removal (requires Windows Media Player)

Publications:

Initial Fluvial Response to the Removal of Oregon’s Marmot Dam