Lake Koshkonong is naturally occurring lake that acts as a reservoir in southern Wisconsin. Its size was augmented by the creation of the Indianford Dam in 1932, making it one of the larger lakes in the state. The lake lies along the Rock River, with the river acting as both the primary inflow and the primary outflow for the lake. Lake Koshkonong begins 5.5 mi (8.9 km) downriver from Fort Atkinson, with the large majority of the lake located in southwestern Jefferson County. Small portions of the lake extend into southeastern Dane and northern Rock counties.