The Kachess River flows into the lake from the north, and out from the south.

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.

[2] As a storage reservoir, Kachess Lake's active capacity is 239,000 acre⋅ft (295 million m3).