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
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)
Turbines6 [1]
Installed capacity72 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]

Embankment dam large artificial dam

An embankment dam is a large artificial dam. It is typically created by the placement and compaction of a complex semi-plastic mound of various compositions of soil, sand, clay, or rock. It has a semi-pervious waterproof natural covering for its surface and a dense, impervious core. This makes such a dam impervious to surface or seepage erosion. Such a dam is composed of fragmented independent material particles. The friction and interaction of particles binds the particles together into a stable mass rather than by the use of a cementing substance.

Gravity dam

A gravity dam is a dam constructed from concrete or stone masonry and designed to hold back water by primarily using the weight of the material alone to resist the horizontal pressure of water pushing against it. Gravity dams are designed so that each section of the dam is stable, independent of any other dam section.

Saint Louis River river in Minnesota and Wisconsin, United States of America

The Saint Louis River is a river in the U.S. states of Minnesota and Wisconsin that flows into Lake Superior. The largest U.S. river to flow into the lake, it is 192 miles (309 km) in length and starts 13 miles (21 km) east of Hoyt Lakes, Minnesota. The river's watershed covers 3,634 square miles (9,410 km2). Near the Twin Ports of Duluth, Minnesota and Superior, Wisconsin, the river becomes a freshwater estuary. The lower St. Louis is the only river in the state with whitewater rafting opportunities.



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]

Great Northern Railway (U.S.) defunct American Class I railway company (1889–1970)

The Great Northern Railway was an American Class I railroad. Running from Saint Paul, Minnesota, to Seattle, Washington, it was the creation of 19th-century railroad entrepreneur James J. Hill and was developed from the Saint Paul & Pacific Railroad. The Great Northern's (GN) route was the northernmost transcontinental railroad route in the U.S.

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]

Minnesota State Highway 210 highway in Minnesota

Minnesota State Highway 210 (MN 210) is a state highway in west-central, central, and northeast Minnesota, which runs from North Dakota Highway 210 (ND 210) at the North Dakota state line, and continues east to its eastern terminus at its intersection with MN 23 in Duluth near the Saint Louis River.

United States Army Corps of Engineers federal agency under the Department of Defense and a major Army command

The United States Army Corps of Engineers (USACE) is a U.S. federal agency under the Department of Defense and a major Army command made up of some 37,000 civilian and military personnel, making it one of the world's largest public engineering, design, and construction management agencies. Although generally associated with dams, canals and flood protection in the United States, USACE is involved in a wide range of public works throughout the world. The Corps of Engineers provides outdoor recreation opportunities to the public, and provides 24% of U.S. hydropower capacity.

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 2017-07-30.
  2. 1 2 3 4 5 "CorpsMap: National Inventory of Dams". United States Army Corps of Engineers. October 2016. Retrieved 29 Jul 2017.
  3. 1 2 Peterson, Jana; Lund, Jamie (2017-06-22). "Carlton County rebuilds smarter after devastating 2012 flood". Pine Journal. Retrieved 2017-07-30.
  4. "Thomson Reservoir". Minnesota Power: Reservoirs and Recreation. Retrieved 2017-07-30.
  5. "2015 Integrated Resource Plan" (PDF). Minnesota Power. September 1, 2015.
  6. "Thomson Dam, Thomson, MN". John A. Weeks III. Retrieved 2017-07-30.
  7. "Listing of Minnesota Hydropower Facility Sites" (PDF). Minnesota Department of Natural Resources. 2017. Retrieved 2017-07-30.
  8. "Hometown Hydropower: History". Minnesota Power. 1979-04-23. Retrieved 2017-07-30.
  9. "Minnesota Power invests in safety improvements to hydro system". Minnesota Power is an ALLETE Company. 2017-01-20. Retrieved 2017-07-30.