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Distribution, thickness, and volume of fine-grained sediment from precipitation of metals from acid-mine waters in Keswick Reservoir, Shasta County, California

Sedimentation History

The sedimentation history in the Spring Creek Arm and in Keswick Reservoir can be divided into three phases: (1) prior to the closing of Keswick Dam in January 1950, (2) between January 1950 and the construction of the Spring Creek Debris Dam in 1963 (Prokopovich, 1991), and (3) after 1963.

Prior to the construction of Keswick Dam, the Spring Creek watershed drained the Spring Creek Arm and the Iron Mountain region, joining the Sacramento River, which flowed freely through a narrow channel in the vicinity of the town of Keswick. Pre-mining sediments were locally derived and largely composed of granitic boulders and cobbles in a sandy matrix, as found in test borings prior to construction of the Spring Creek Debris Dam. Most of the sediments entering lower Spring Creek were flushed into the Sacramento River during major winter storms and then entered into the normal Sacramento River sedimentary system. These sediments included mining wastes and smelter slag, some of which still lines the banks of the Spring Creek Arm of Keswick Reservoir.

The closure of Keswick Dam in 1950 flooded the lower Spring Creek area. This event led to the rapid construction of a delta in lower Spring Creek between 1950 and 1963 that was composed of alluvium, smelter wastes, and mining-related debris flushed down Spring Creek into the reservoir. By 1960, the length of the subaerial delta was about 850 m, ranging from 0.1 to 8.6 m in thickness, and averaging 4.2 m (Prokopovich, 1991). The shallowest part of the Spring Creek Delta was excavated during construction of the tailrace channel for the Spring Creek Power Plant; at least some of the excavated material was used in the construction of the Spring Creek Debris Dam (Prokopovich, 1991).

After completion of the Spring Creek Debris Dam in 1963, sedimentation into the lower Spring Creek Arm of Keswick Reservoir was largely derived from metals precipitated out of the acid-mine drainage waters entering the reservoir via Spring Creek. The resulting reddish-brown mud comprises most of the sediment sampled from the present sediment accumulations in the arm. Within these muds are occasional layers of sandy to silty sediment, probably derived during times of high flow or overflow from the Spring Creek Debris Dam. The sediments that now occupy the arm consist largely of chemically precipitated materials of distinctly fine-grain size; these were the focus of the 1993 surveying effort by the USGS.