ABANDONED MINE RECLAMATION OF SEDIMENT TAILINGS DAMS AT KING SOLOMON MINE, KLAMATH NATIONAL FOREST, CALIFORNIA

Abandoned mine inventory and assessment on the Klamath National Forest has identified several reclamation and cleanup challenges. Removal of three remnant sediment tailings dams and stream channel stabilization at King Solomon Mine is an ongoing restoration project. King Solomon Mine was the third largest open pit gold mine within the Forest, and was active up until 1941. Tailings sediment was piled behind three valley dams that were as high as 15.8 meters. Today, the remnant tailings extend for 347 meters upstream of the lowest dam and are preserved in terraces along the creek. The dams appear to have been breached by stream flows between 1944 and 1955. The remaining dam abutments constrict the stream channel, and stream undercutting is threatening further failure and collapse. Characterization of the metals composition and leaching potential of both tailings sediment and dam soils is critical to the engineering design of stabilized dam slopes, stabilization of channel banks, waste area design and assessing effects of sedimentation on downstream aquatic habitat. Results of testing indicates that dam soils and tailings are non-toxic and similar in metals composition to background soils that are not mineralized and undisturbed by mining. Slope stability assessment indicates that further failure of the dam remnants could release 6,116 cubic meters of sediment into the channel. Debris flows from dam collapse could scour 4.6 kilometers of stream channel, destroying important fish habitat for steelhead in Matthews Creek, and chinook and coho salmon in the South Fork Salmon River. Dam removal and channel stabilization measures are proposed to restore the stream channel to a stable configuration. Further analysis is underway to estimate tailings sediment erosion during the period of channel adjustment after dam removal. Pre- and post-project channel profiles and cross sections will be monitored to determine channel migration and downcutting after restoration is completed.