Gibson Dam
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Gibson Dam is a concrete arch dam on the
Sun River The Sun River (also called the Medicine River) is a tributary of the Missouri River in the Great Plains, approximately 130 mi (209 km) long, in Montana in the United States. Geography It rises in the Rocky Mountains in two forks, the ...
, a tributary of the
Missouri River The Missouri River is a river in the Central United States, Central and Mountain states, Mountain West regions of the United States. The nation's longest, it rises in the eastern Centennial Mountains of the Bitterroot Range of the Rocky Moun ...
, about west of
Great Falls, Montana Great Falls is the List of cities and towns in Montana, third most populous city in the U.S. state of Montana and the county seat of Cascade County, Montana, Cascade County. The population was 60,442 according to the 2020 United States census, 2 ...
in the
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. Located on the eastern edge of the
Rocky Mountains The Rocky Mountains, also known as the Rockies, are a major mountain range and the largest mountain system in North America. The Rocky Mountains stretch in great-circle distance, straight-line distance from the northernmost part of Western Can ...
, the dam was built by the U.S. Bureau of Reclamation (USBR) between 1926 and 1929 as part of the Sun River Project to develop about of irrigated land in the Sun River Valley.


Specifications

The dam is a high and long arch gravity structure, with a base thickness of tapering to at the crest. As a whole the dam contains about of concrete. The impounded water forms
Gibson Reservoir Gibson Reservoir is a man-made body of water located just south of the Bob Marshall Wilderness, northwest of Augusta, Montana. It forms part of the county line between Teton and Lewis & Clark counties. The reservoir is formed by the concrete ...
, which can store up to at full water levels of . When full, the reservoir covers , with a shoreline of roughly and a maximum depth of . Water releases are controlled by two sets of outlets: three release valves at the base of the dam have a combined capacity of , while a gated tunnel spillway, controlled by six
radial gate The Tainter gate is a type of radial arm floodgate used in dams and canal locks to control water flow. It is named for its inventor, the Wisconsin structural engineer Jeremiah Burnham Tainter. Tainter, an employee of the lumber firm Knapp, ...
s, can pass a maximum of . Gibson Dam's main function is to capture spring snowmelt and release it during summer for the irrigation for about of land on the north side of the Sun River and south of the river. Water released from Gibson travels downstream for about before it is intercepted by the Sun River Diversion Dam, which diverts water into the Pishkun Supply Canal, which sends water to the offstream Pishkun Reservoir. The Sun River Slope Canal System takes water from Pishkin Reservoir to serve the irrigated lands on the north side of the river. The headworks for the Fort Shaw Canal, which serves the lands south of the river, are located further downstream and take water directly from the river. The lands north of the river comprise the Greenfields Irrigation District, while the south side is run by the Fort Shaw Irrigation District.


History

On September 26, 1906, the
Department of the Interior The United States Department of the Interior (DOI) is an executive department of the U.S. federal government responsible for the management and conservation of most federal lands and natural resources. It also administers programs relatin ...
authorized the USBR's Sun River Project, under pressure from local residents, namely those of Great Falls, who wanted the irrigation of lands east of the Rocky Mountains along the Sun and Teton Rivers. Early proposals included the diversion of streams from west of the
Continental Divide A continental divide is a drainage divide on a continent such that the drainage basin on one side of the divide feeds into one ocean or sea, and the basin on the other side either feeds into a different ocean or sea, or else is endorheic, not ...
to augment the arid region's water supply, but eventually the project was pared down to comprise two storage dams on the Sun River and a tributary, Willow Creek; two off-stream reservoirs and a diversion dam; and seven main canals. As early as 1889, the Gibson Dam site – located in a narrow mountain gap a few miles above the mouth of the Sun River canyon – had been identified by U.S. surveyor Herbert Wilson as an excellent location to develop water storage on the Sun River. More detailed studies in 1911 by the USBR (then the U.S. Reclamation Service) confirmed Wilson's idea, and the first plans for a dam were drafted around 1920. However, it was not until September 1926 when the $1.5 million main construction contract was given to
Utah Construction Company The Utah Construction Company was a construction company founded by Edmund Orson Wattis Jr., Warren L. Wattis and William. H. Wattis in 1900. History The Wattis Brothers received funding from David Eccles, Thomas Dee, Joseph Clark and Jam ...
. The dam was one of the first to be built using the trial-load method, which relies primarily on mathematical calculations to determine the pressure of water on the structure, and thus the dam's final design. This method allowed the USBR to save an estimated of concrete, lowering costs significantly, since otherwise a considerable safety margin would have had to be built into the dam. Actual construction started in December 1926, with Albert E. Paddock as project superintendent and Ralph Lowry as construction engineer. Preliminary excavations and site clearing were followed by the placement of a wooden flume to divert the river before concrete pouring in the foundation work could begin. When the foundations were complete, the flume was closed and water allowed to pass through outlet holes in the base of the dam. Concrete for the dam was mixed at a site roughly downstream and transported to the construction site via a light railway. The concrete was then placed via buckets transported by a cableway system, supported by two high towers. Two penstocks were installed on the south side of the dam and of transmission lines were built connecting the dam and
Augusta, Montana Augusta is an unincorporated community and census-designated place (CDP) in Lewis and Clark County, Montana, United States. The population was 309 at the 2010 census and rose to 316 in the 2020 census. The most accepted version in the naming of ...
in preparation for future hydroelectric generation. However, after determining that the future reservoir would not have enough capacity to sustain power production, USBR sealed the penstocks and the power lines fell into disuse. The dam was completed and the first water stored in December 1929 after three years of construction. In 1938, gates were installed in the spillway, increasing the reservoir's storage capacity by 20 percent. At least three people were killed on the project, including Paddock and an unknown workman; the latter fell from a height and struck the former on April 20, 1929. In the early phases of construction in 1926, a laborer, M. G. Miller, died from injuries sustained from a dynamite explosion.


Flood control

By halting the Sun River's spring
freshet The term ''freshet'' is most commonly used to describe a snowmelt, an annual high water event on rivers resulting from snow and river ice melting. Description A spring freshet can sometimes last several weeks on large river systems, resulting ...
, Gibson Dam has prevented an estimated $3,044,000 in flooding damage between 1950 and 1999. In 1964, western Montana experienced one of the greatest flooding events in its recorded history, when record snowpack followed by heavy early spring rains caused more than $438 million (2011 dollars) of damage in the Flathead and Missouri River basins. On June 8, after reports of at least three dam failures on tributaries of the Missouri sent local residents into a panic, there were rumors that the Gibson Dam had failed. A
U.S. Forest Service The United States Forest Service (USFS) is an agency within the U.S. Department of Agriculture. It administers the nation's 154 national forests and 20 national grasslands covering of land. The major divisions of the agency are the Chief's ...
pilot was sent to investigate and found that water was overtopping the dam by over . The USBR later estimated the peak flow over the dam at , of which at least half was over the crest. After this event the dam was retrofitted so that it could be safely overtopped by up to of water.


Hydroelectric power proposal

The Gibson Dam Hydroelectric Company "Gibson Hydro" is a two-member partnership proposing a renewable energy facility on the Gibson Dam. Tollhouse Energy of Bellingham, WA a small-business hydropower developer and Greenfields Irrigation District of Fairfield, MT a farmer-managed water utility, are proposing the addition of a 15
megawatt The watt (symbol: W) is the unit of Power (physics), power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantification (science), quantify the rate of Work ...
(MW) powerhouse at the base of the dam. The powerhouse would generate electricity from irrigation water releases with an estimated annual production of 42.9 million
kilowatt hour A kilowatt-hour ( unit symbol: kW⋅h or kW h; commonly written as kWh) is a non-SI unit of energy equal to 3.6 megajoules (MJ) in SI units, which is the energy delivered by one kilowatt of power for one hour. Kilowatt-hours are a commo ...
s.


Climate


See also

*
List of dams and reservoirs in Montana A list is a set of discrete items of information collected and set forth in some format for utility, entertainment, or other purposes. A list may be memorialized in any number of ways, including existing only in the mind of the list-maker, but ...


References


External links


Gibson Reservoir live water levels
{{Webarchive, url=https://web.archive.org/web/20120813052046/http://www.usbr.gov/gp-bin/arcweb_gibr.pl , date=2012-08-13 Buildings and structures in Lewis and Clark County, Montana Buildings and structures in Teton County, Montana Dams in Montana United States Bureau of Reclamation dams Arch dams Dams completed in 1929