# Hebgen Dam

Hebgen Dam is a concrete-core earthen embankment dam in the western United States, located on the Madison River in southwestern Montana. The dam is 85 feet (26 m) tall and 721 feet (220 m) in length; its purpose is to store and regulate water for other downstream reservoirs and hydroelectric power plants. Montana Power Company originally built the dam, PPL Corporation purchased it in 1997 and sold it to NorthWestern Corporation in 2014.

## History

Hebgen Dam was built across the Madison River in 1914 by Montana Power Company to create Hebgen Lake.
During the 7.5 magnitude 1959 Hebgen Lake earthquake in mid-August, the dam was damaged, primarily because of such intense ground movement that water surged over the dam crest four different times, but it was fixed several weeks later. The epicenter of the quake was determined to be 20 miles (32 km) beneath the bottom of Hebgen Lake. Seismologists reported it to be the fourth largest quake recorded in the United States up to that time.
On August 30, 2008, two of the dam's four hydraulic gates failed, releasing 3,400 cubic feet (96 m$^3$) per second of water into the Madison River. The normal discharge of the dam is 900 cubic feet (25 m$^3$) per second and the gate failure caused a 1-foot rise in the river.
On November 30, 2021, the hydraulic gates failed again causing downstream flows to rapidly drop. Nearby locals quickly organized an effort to rescue stranded fish trapped in pools in the side channels. An ongoing lawsuit against Northwestern Energy (as of 2024) claims that NWE failed in its requrirement to keep streamflows above 150 CFS.

## Climate

Climate data for Hebgen Dam, Montana, 1991–2020 normals, 1904–2020 extremes: 6489ft (1978m) :
| Month | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | Year |
| Record high °F (°C) | 45 (7) | 49 (9) | 60 (16) | 75 (24) | 88 (31) | 96 (36) | 98 (37) | 94 (34) | 93 (34) | 82 (28) | 62 (17) | 51 (11) | 98 (37) |
| Mean maximum °F (°C) | 35.2 (1.8) | 40.5 (4.7) | 51.4 (10.8) | 62.9 (17.2) | 74.3 (23.5) | 81.6 (27.6) | 88.0 (31.1) | 87.3 (30.7) | 81.8 (27.7) | 68.2 (20.1) | 49.1 (9.5) | 36.7 (2.6) | 88.5 (31.4) |
| Mean daily maximum °F (°C) | 22.9 (−5.1) | 27.9 (−2.3) | 38.4 (3.6) | 46.8 (8.2) | 58.4 (14.7) | 68.4 (20.2) | 78.7 (25.9) | 78.0 (25.6) | 67.7 (19.8) | 50.7 (10.4) | 34.0 (1.1) | 22.7 (−5.2) | 49.6 (9.7) |
| Daily mean °F (°C) | 13.2 (−10.4) | 16.5 (−8.6) | 26.1 (−3.3) | 35.0 (1.7) | 45.4 (7.4) | 53.7 (12.1) | 61.7 (16.5) | 60.7 (15.9) | 52.4 (11.3) | 39.4 (4.1) | 26.0 (−3.3) | 14.3 (−9.8) | 37.0 (2.8) |
| Mean daily minimum °F (°C) | 3.5 (−15.8) | 5.1 (−14.9) | 13.8 (−10.1) | 23.3 (−4.8) | 32.3 (0.2) | 38.9 (3.8) | 44.6 (7.0) | 43.4 (6.3) | 37.2 (2.9) | 28.2 (−2.1) | 17.9 (−7.8) | 5.8 (−14.6) | 24.5 (−4.2) |
| Mean minimum °F (°C) | −23.9 (−31.1) | −24.1 (−31.2) | −10.8 (−23.8) | 6.8 (−14.0) | 20.6 (−6.3) | 29.5 (−1.4) | 36.6 (2.6) | 35.2 (1.8) | 26.2 (−3.2) | 13.1 (−10.5) | −1.4 (−18.6) | −18.5 (−28.1) | −29.4 (−34.1) |
| Record low °F (°C) | −47 (−44) | −60 (−51) | −38 (−39) | −20 (−29) | 8 (−13) | 19 (−7) | 25 (−4) | 12 (−11) | 6 (−14) | −9 (−23) | −35 (−37) | −43 (−42) | −60 (−51) |
| Average precipitation inches (mm) | 3.06 (78) | 2.76 (70) | 2.17 (55) | 2.28 (58) | 3.14 (80) | 2.99 (76) | 1.62 (41) | 1.38 (35) | 1.59 (40) | 2.08 (53) | 2.49 (63) | 3.07 (78) | 28.63 (727) |
| Average snowfall inches (cm) | 43.80 (111.3) | 33.70 (85.6) | 20.60 (52.3) | 7.70 (19.6) | 1.20 (3.0) | 0.00 (0.00) | 0.00 (0.00) | 0.00 (0.00) | 0.00 (0.00) | 4.10 (10.4) | 20.90 (53.1) | 45.20 (114.8) | 177.2 (450.1) |
| Source 1: NOAA ||||||||||||||
| Source 2: XMACIS2 (records & monthly max/mins) ||||||||||||||