Tieton Dam, on the Tieton River about 40 miles west of Yakima, is an earthfill structure with a concrete core wall that extends from the crest to about 100 feet below the riverbed. The dam is 319 feet high and contains 2,049,000 cubic yards of material. The reservoir’s active capacity is 198,000 acre-feet.
The geologic structure of the basin has a base of shale and sandstone sediments which were subsequently overlain by the basalt flows of the Columbia River lava. As the Cascade Mountains uplifted, stream erosion cut deep canyons through the basalt and underlying rock. Subsequent to the uplift, lava flows completely covered the upper ends of the stream valleys and partly filled sections of the stream with andesite. The andesite in the canyons was later eroded away leaving the upper end of the valley still deeply buried. Glacial material and soil covers much of the valley floors and occassional terraces line the valley side.