Thomson Dam (Minnesota)

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Thomson Dam
Thomson Dam, Minnesota.jpg
Downstream face of the main Thomson Dam on the St. Louis River in 2017
USA Minnesota relief location map.svg
Red pog.svg
Location of Thomson Dam in Minnesota
Location Carlton County, Minnesota, U.S.
Coordinates 46°39′59.10″N92°24′25.80″W / 46.6664167°N 92.4071667°W / 46.6664167; -92.4071667 Coordinates: 46°39′59.10″N92°24′25.80″W / 46.6664167°N 92.4071667°W / 46.6664167; -92.4071667
Purpose Power
Construction began1905
Opening date1907
1914-48 expanded [1]
2012 damaged
2014 reconstructed
Built by Great Northern Railway
Owner(s) Minnesota Power
Dam and spillways
Type of dam Earth Embankment, Concrete Gravity, Arch
Impounds Saint Louis River
HeightMain: 15 ft (4.6 m) [2]
Canal: 45 ft (14 m)
LengthMain: 1,600 ft (490 m) [2]
Canal: 3,500 ft (1,100 m)
Spillways 2
Spillway typegated
Spillway capacity60,000 cu ft/s (1,700 m3/s) [3]
CreatesThomson Reservoir
Total capacity4,352 acre⋅ft (5,368,000 m3) [2]
Catchment area 9,154 sq mi (23,710 km2) [2]
Surface area649 acres (263 ha) [4]
Thomson Hydro
Coordinates 46°39′17.91″N92°20′1.032″W / 46.6549750°N 92.33362000°W / 46.6549750; -92.33362000
Hydraulic head 375 ft (114 m)
Turbines 6 [1]
Installed capacity 72 MW [1]
Annual generation 280 GWh [5]

Thomson Dam, also known as the Thomson Hydro Station [1] or Thomson Water Project, [6] is an embankment and concrete gravity dam on the Saint Louis River near the town of Thomson in northeastern Minnesota, United States. It consists of a 1600-foot (488 m) long primary structure and multiple supplementary dams which, together with precambrian rock outcrops known as the Thomson formation, impound the river to create Thomson Reservoir.


The tallest dam in the complex is 51.6 feet (16 m) and the longest is 3500 feet (1067 m). A series of gate houses, a canal, forebay, and underground penstocks supply a hydropower plant located 3 miles away in Jay Cooke State Park. With an installed capacity of 72 MW and an annual generation of approximately 280 GWh, the Thomson project is the largest hydroelectric facility in the state. [7]


Thomson Dam was completed in 1907 by Great Northern Power, an operating division of the Great Northern Railway. The generating station was expanded in 1914 with the addition of Unit 4. [1] Unit 5 was added in 1918 and Unit 6 in 1948. Railroad tracks built into the generator floor allowed for installation and maintenance of the equipment. The complex was later transferred to the Saint Louis Power Company. Today it is owned by Minnesota Power, a division of Allete, Inc. [8]

Heavy rains in June 2012 created an historic flood in the region which overtopped the dam, breached the forebay canal and severely damaged the hydroelectric station. Following $90 million in reconstruction and upgrades, including the addition of a new emergency spillway, the facility came back online in November, 2014. Additional upgrades will continue through 2018, including removal of the original 46kV transmission line equipment in favor of other, higher voltage equipment that was added later. [3] [9]


The most visible part of Thomson Dam is the primary structure straddling the Saint Louis River channel near Minnesota State Highway 210. However, the Thomson Project is actually composed of multiple dams and control structures, several of which have been rebuilt and merged over the years. Today the United States Army Corps of Engineers National Inventory of Dams (NID) counts 18 structures as part of the complex, with 14 formally listed as separate. [2]

Thomson Dam - NID Registered Structures
Dam IDOther IDNameHeightWidthType
MN00604Thomson Dam15 ft (4.6 m)1,600 ft (490 m) Embankment and concrete gravity
MN00604S010Thomson Canal Dam45 ft (14 m)3,500 ft (1,100 m) Embankment
MN83020S011Thomson Dam #1-1/210 ft (3.0 m)90 ft (27 m) Embankment
MN83021S001Thomson Dam #2A, 2B23 ft (7.0 m)530 ft (160 m)na
MN83022S012Thomson Dam #2-1/29 ft (2.7 m)130 ft (40 m) Concrete gravity
MN83023S002Thomson Dam #3
(Nos 2-3/4, 3, 3A, 4, 4A)
38 ft (12 m)1,322 ft (403 m) Concrete gravity
MN83024S003Thomson Dam #523 ft (7.0 m)100 ft (30 m) Concrete gravity
MN83025S013Thomson Dam #5-1/223 ft (7.0 m)115 ft (35 m) Concrete gravity
MN83026S004Thomson Dam #651.6 ft (15.7 m)125 ft (38 m) Concrete arch
MN83027S005Thomson Dam #812 ft (3.7 m)100 ft (30 m)na
MN83028S006Thomson Dam #911 ft (3.4 m)100 ft (30 m) Concrete gravity
MN83029S007Thomson Dam #1011 ft (3.4 m)80 ft (24 m) Concrete gravity
MN83030S008Thomson Dam #11
(Nos. 11, 11-1/2 and Upper Gate House)
17 ft (5.2 m)365 ft (111 m) Concrete gravity
MN83031S009Thomson Dam #1212 ft (3.7 m)450 ft (140 m) Embankment

See also

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  1. 1 2 3 4 5 "Thomson Hydro Station". Minnesota Power: Our System. Retrieved July 30, 2017.
  2. 1 2 3 4 5 "CorpsMap: National Inventory of Dams". United States Army Corps of Engineers. October 2016. Retrieved July 29, 2017.
  3. 1 2 Peterson, Jana; Lund, Jamie (June 22, 2017). "Carlton County rebuilds smarter after devastating 2012 flood". Pine Journal. Retrieved July 30, 2017.
  4. "Thomson Reservoir". Minnesota Power: Reservoirs and Recreation. Retrieved July 30, 2017.
  5. "2015 Integrated Resource Plan" (PDF). Minnesota Power. September 1, 2015.
  6. "Thomson Dam, Thomson, MN". John A. Weeks III. Retrieved July 30, 2017.
  7. "Listing of Minnesota Hydropower Facility Sites" (PDF). Minnesota Department of Natural Resources. 2017. Retrieved July 30, 2017.
  8. "Hometown Hydropower: History". Minnesota Power. April 23, 1979. Retrieved July 30, 2017.
  9. "Minnesota Power invests in safety improvements to hydro system". Minnesota Power is an ALLETE Company. January 20, 2017. Retrieved July 30, 2017.