Kachess Dam is an earthfill structure located on the Kachess River about 2 miles northwest of Easton. This dam, 115 feet high with a volume of 200,000 cubic yards, was built at the lower end of a natural lake, and created a reservoir with an active capacity of 239,000 acre-feet constructed over a natural lake having unknown dead storage capacity.
The discharge channel for Kachess Reservoir is 2,877 feet long and was constructed from the natural lake to the intake structure of the dam’s outlet works. This was done so the natural lake could be used for storage. The first 1,182 feet consists of an open cut inlet channel leading to a 9 foot diameter tunnel for 1,393 feet and then 302 feet of open channel to the intake structure. Significant siltation has occurred in the discharge channel causing a restriction in passing the necessary water downstream.
In 1996, sediment was removed from the inlet channel and a new channel was excavated along the existing tunnel. In addition, a new intake structure was constructed, the steel bridge from the dam to the intake structure replaced, and the outlet works conduit lined with a minimum of 4-inch thick reinforced concrete overlay. The channel improvements were accomplished under Phase II of the Yakima River Basin Water Enhancement Project; Title XII of the Act of October 31, 1994. The intake structure and outlet conduit work was done as a part of the Bureau of Reclamation’s Operations and Maintenance Modifications Program.
A glacial till moraine extends across the entire Kachess River valley and comprises the foundation and abutments of the dam. The glacial till is composed of a heterogeneous mixture of clay, silt, sand, gravel, cobbles, and boulders. These materials were used as the borrow for construction of the impervious and pervious zones of the dam.
Preconstruction investigations suggest the thickness of the moraine ranges from 45 to 60 feet between dam stations 8+00 and 13+20. From dam station 13+20 to the left end of the dam at station 14+00, the moraine thickens rapidly to about 100 feet. The thickness of the moraine between dam stations 0+00 and 8+00 probably ranges from about 80 to 100 feet, and possibly up to 200 feet, based on the site topography and depth of the reservoir. Site investigations during original construction showed the moraine in the foundation and borrow areas to be comprised of lenses of fine materials (blue, brown, and gray clays), hardpan (fines, sand, gravel, and plus 3-inch material variably cemented with calcareous or iron-based agents), and various uncemented combinations and gradations of sand, gravel and plus 3-inch material.