Lake Koshkonong

Lake Koshkonong is a reservoir in southern Wisconsin. It lies along the Rock River, 5.5 mi (8.9 km). down-river from Fort Atkinson, primarily in southwestern Jefferson County, although small portions of the lake extend into southeastern Dane and northern Rock counties.

Lake Koshkonong shares its name with Fort Koshkonong, a fort of some importance during the Black Hawk War. It is a natural lake, and at the time the fort was active, before the Rock River was dammed, the area was a cattail marsh with the Rock River running through the middle. The Indianford Dam several miles down the Rock River from Lake Koshkonong has made it one of the larger lakes in the state of Wisconsin (10,500 acres (42 km2)) but it remains very shallow with an average depth of six feet (2 m). The dam dates from 1932, thirteen feet high (4 m) and with a length of 500 ft (150 m) at its crest. Maximum capacity of the reservoir is 107,000 acre·ft (132,000,000 m3). Both dam and reservoir are owned and operated by Rock County.

The lake borders on its east the town of Koshkonong, and on its north, Sumner.