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Savage Rapids Dam, Rogue River near Grants Pass, Oregon

Savage Rapids Dam

Savage Rapids Dam

Savage Rapids Dam is located in south-central Oregon, on the Rogue River, just 5 miles upstream from the town of Grants Pass. The annual mean flow of the Rouge River is 3,372 ft3/s. With a total drainage area of 2,459 mi2, the mean annual runoff is 19 inches. The highest peak flow recorded was 152,000 ft3/s on December 23, 1962 and the lowest mean-daily flow recorded was 744 ft3/s. Savage Rapids dam was built in 1921 to divert river flows for irrigation. The dam is 40 feet tall and creates a backwater pool that extends ½ mile upstream during the non-irrigation season and 2 ½ miles upstream during the irrigation season. The reservoir width is relatively narrow, only two to three times greater than the width of the river (Bountry and others, written communication, 2000). Although the dam has fish ladders, these ladders are old, do not meet current regulations, and only allow limited fish passage. Dam removal has been proposed to restore fish passage to natural conditions. The dam would be replaced with two pumping plants that would deliver water to the irrigation canals. A sediment study was requested by the Grants Pass Irrigation District to determine the sediment-related impacts of dam removal.

Sediment Behind Savage Rapids Dam

Sediment Stored Behind Savage Rapids Dam

Sediment Behind Savage Rapids Dam

Stored Sediment Volume

The volume and quality of the reservoir sediments were the critical properties to be determined. Previous estimates of the reservoir sediment volume had ranged from 0.5 million to 1 million yds3. These estimates were based on limited information and seemed large for a reservoir of this size. Because the reservoir is significantly drawn down during the winter flood season (non-irrigation season), river conditions exist in the upper two miles of the reservoir during the period of maximum sediment inflow. Therefore, sediment was not able to significantly deposit along the upper two miles of the reservoir, but it did deposit in the 2-mile reach immediately upstream from the dam where the permanent pool was established. The techniques that were used to determine a more accurate estimate of the sediment volume included a visual inspection of the shoreline during a low reservoir pool, dive team examinations of the reservoir bottom, bathymetric surveying of the reservoir bottom, and core drilling of the reservoir sediments. Based on these more accurate techniques and a knowledge of reservoir operations, the reservoir sediment volume was determined to be only 200,000 yds3. This volume is roughly equivalent to a 2-year sediment supply from the Rogue River. The reservoir sediments were found to consist of 71 percent sand, 27 percent gravel, and 2 percent silt and clay. Chemical testing of the reservoir sediments did not find any contaminants in concentrations significantly greater than natural background levels. These sediments would not pose any hazard to water quality, fish and wildlife, or human uses if released downstream of the dam (Bountry and others, written communication, 2000). Without drill-hole data and without an analysis of reservoir operations, the previous estimates of the reservoir sediment volume were significantly too high. Drill hole data from the deep areas of the reservoir were necessary to accurately determine the reservoir sediment volume and chemical composition.

A bathymetric survey of the downstream river channel was performed to develop river cross sections. The HEC-6t sediment transport model (Thomas, 1996) was used to simulate the erosion of sediments from the reservoir and their transport downstream. Model results indicate that the majority of sediments would be eroded from the reservoir within the first year following dam removal. The downstream transport of this sediment is dependent on the magnitude and duration of river flows following dam removal.

Cross Section Adjustment

Change in Cross Section

References

Bountry, J.A.; Randle, T.J.; Link, R.A.; Yahnke, J.; Bullard, K.L.; Black, R., September 2000, written communication, draft report in progress: Savage Rapids Dam Sediment Evaluation Study, Rogue River, Oregon, U.S. Department of the Interior, Bureau of Reclamation, Denver, Colorado.

Thomas, W.A., March 1996, HEC 6t Sedimentation in stream networks, User's Manual, Version: h6tr514, Mobile Boundary Hydraulics, Vicksburg, Mississippi.