EVALUATION OF SEDIMENTS IMPOUNDED BY GOLD RAY DAM on THE ROGUE RIVER IN JACKSON COUNTY, OREGON: IMPLICATIONS FOR DAM REMOVAL

A sediment study of the reservoir impounded by Gold Ray Dam on the Rogue River in Jackson County, Oregon was conducted using SONAR data along with the recovery of twelve sediment cores. The SONAR data was used to determine the approximate pre-dam river bottom and to identify submerged sediment bars in the inundated areas of the reservoir. A calculated volume of ~400,000 cubic yards of sediment occurs in the currently inundated area of Gold Ray Reservoir. The sediment cores provided insights into the grain size of sediment impounded by the reservoir. The amount of gravel in the reservoir is ~60,000 cubic yards (~15 percent). The remaining amount of sediment is dominated by sand (~75 percent) with a smaller proportion of mud (~10 percent).

Overall, the sediment volumes calculated above for Gold Ray Reservoir are less than expected, especially with annual sediment loads of 100,000 cubic yards per year estimated for Savage Rapids Dam downstream of this locality. There are three possibilities that may explain the lack of sediment in the reservoir: (1) sediment was deposited by a prograding delta that filled in the inundated area of the floodplain upstream from Gold Ray Dam, (2) the gradient of the Rogue River in this stretch is less, and therefore sediment yields are less, and/or (3) the extraction of gravels upstream decreased the availability of sediments delivered into the reservoir. The amount of sediment estimated to have been deposited on this inundated floodplain may be as much as 1,800,000 cubic yards.

This study also reveals the presence of the original log crib dam built in 1904 just upstream of the concrete Gold Ray Dam erected in 1941. SONAR data indicates that a sediment wedge has been deposited upstream from the submerged log crib dam. On river left, sediments have topped over the submerged log crib dam have been deposited between the log crib dam and the concrete dam. In contrast, the center and river right areas appear to have minimal sedimentation between the log crib and concrete dams with water depths in some places of up to 30 feet.

This sediment study is essential to the planned removal of Gold Ray Dam from the Rogue River, providing valuable information for developing sediment transport models. Gold Ray Dam is slated for removal in August 2010.