Madison Dam

45°28′12″N 111°38′18″W / 45.47012°N 111.63831°W / 45.47012; -111.63831PurposeHydroelectric, Flood Control, Water Supply, RecreationOpening date1906Owner(s)PPL MONTANA, LLCOperator(s)NorthWestern CorporationDam and spillwaysType of damTimber cribHeight39 ft (12 m)Length257 ft (78 m)Spillway capacity7,700 cubic feet per second (220 m³/s)ReservoirCreatesEnnis LakeTotal capacity42,053 acre⋅ft (51,872,000 m³)Maximum length2.6 mi (4.2 km)Normal elevation4,820 ft (1,469 m)^[1]Installed capacity9 MW

Madison Dam is a hydroelectric dam on the Madison River in Madison County, Montana, in the southwestern part of the state.

The timber-crib dam was constructed in 1906 as a replacement for a similar 1901 dam and powerhouse on the same site. The dam is 39 feet (12 m) high and 257 feet (78 m) long at its crest,^[2] placed into the narrows of Bear Trap Canyon. As one of eight PPL Montana hydro projects, it has a generating capacity of 9 megawatts (12,000 hp) in a run-of-the-river configuration.^[3] Montana Power Company acquired the dam in 1912 as part of a merger, PPL Corporation purchased it in 1997 and sold it to NorthWestern Corporation in 2014.

The reservoir it creates, Ennis Lake, is 2.6 miles (4.2 km) long and has a maximum storage capacity of 42,053 acre-foot (51,872,000 m³).^[2] The lake is relatively shallow and warms significantly during the summer, which tends to decrease fish populations.^[4] The lake supports recreational fishing for brown trout and rainbow trout, camping, boating, and white-water rafting in Bear Trap Canyon downstream (north) of the dam.^[5]

• Historic American Engineering Record (HAER) No. MT-179, "Madison Hydroelectric Facility, 950 Ennis Lake Road, Ennis, Madison County, MT", 13 photos, 19 data pages, 1 photo caption page