|Location||Kittitas County, Washington, United States|
|Type||reservoir, natural lake|
|Primary inflows||Kachess River|
|Primary outflows||Kachess River|
|Catchment area||63 sq mi (160 km2)|
|Basin countries||United States|
|Max. length||9.6 mi (15.4 km)|
|Max. width||1.2 mi (1.9 km)|
|Water volume||238,000 acre feet (294,000,000 m3)|
|Surface elevation||2,254 ft (687 m)|
Kachess Lake is a lake and reservoir along the course of the Kachess River in Washington state, US. The upper part of the lake, north of a narrows, is called Little Kachess Lake. The Kachess River flows into the lake from the north, and out from the south. Kachess Lake is the middle of the three large lakes which straddle Interstate 90 north of the Yakima River in the Cascade Range. The other two are Cle Elum Lake, the easternmost which is also north of I-90 and Keechelus Lake, the westernmost, which is south of I-90.
Kachess Lake is part of the Columbia River basin, the Kachess River being a tributary of the Yakima River, which is tributary to the Columbia River.
The lake is used as a storage reservoir for the Yakima Project, an irrigation project run by the United States Bureau of Reclamation. Although a natural lake, Kachess Lake's capacity and discharge is controlled by Kachess Dam, a 115-foot (35 m) high earthfill structure built in 1912. 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, approximately 1800 feet downstream and at a lower elevation than the original lake outlet. The intent of the lowered outlet works was to put all of the average annual runoff into service by adding an additional 76,000 acre feet of natural lake water. As a storage reservoir, Kachess Lake's active capacity is 239,000 acre feet (295,000,000 m3).
The name Kachess comes from a Native American term meaning "more fish", in contrast to Keechelus Lake, whose name means "few fish".
- Baldwin, E. H. (May 15, 1913). "Construction of the Kachess Dam, Washington". Engineering News. 69 (20): 989–990. Retrieved 13 June 2018.
- Yakima Project Archived 2007-04-30 at the Wayback Machine, United States Bureau of Reclamation.
- Phillips, James W. (1971). Washington State Place Names. University of Washington Press. ISBN 0-295-95158-3.