Clear Creek Dam, a concrete thin-arch structure on the North Fork of the Tieton River about 30 miles southwest of Naches and 48 miles west of Yakima, creates a reservoir with an active capacity of 5,300 acre-feet. The dam is 83 feet high and contains 5,800 cubic yards of concrete. Originally constructed in 1914, the dam was raised 21 feet in 1918 to its present height. Rehabilitation work in 1964 consisted primarily of placing new concrete in the arch section between elevation 2,991.0 and the crest, repairing cracks and poorly consolidated concrete with neoprene and epoxies, and installing protective wire-mesh fences from the abutments to upstream areas.
As the result of investigations conducted in 1987-1989, the Bureau of Reclamation concluded there were horizontal bands of deteriorated concrete in the section replaced in 1964. These conditions could result in sudden failure of the dam. The water level of Clear Lake was immediately lowered to reduce the likelihood of dam failure. Early in 1990, two holes were cut through the dam to further drain the lake to a point that only 230 acre-feet could be stored.
Overwhelming public support was expressed for reconstructing the dam and returning Clear Lake to its original condition, due mostly to the loss of recreational value. Reconstruction was accomplished through local, State, and Federal funding and consisted of converting the dam into a gravity structure by buttressing the arch with a new concrete section immediately downstream. A new 48-inch outlet conduit was installed near the elevation of the breach (2956.5 feet) to permit discharge whenever the lake was below the spillway crest. Clear Lake is now used primarily for recreation and most of the discharge is by the spillway.
The dam foundation is composed of a metamorphosed sill of highly fractured, basic porphyritic rock of Tertiary age, altered at the surface to serpentine and epidote. Field examination showed this rock to be solidly keyed and fused together, competent to support the dam, and highly resistant to erosion.