The Colorado–Big Thompson Project (abbreviated C-BT) is a federal water diversion project in

Colorado Colorado is a U.S. state, state in the Western United States. It is one of the Mountain states, sharing the Four Corners region with Arizona, New Mexico, and Utah. It is also bordered by Wyoming to the north, Nebraska to the northeast, Kansas ...

designed to collect West Slope mountain water from the headwaters of the

Colorado River The Colorado River () is one of the principal rivers (along with the Rio Grande) in the Southwestern United States and in northern Mexico. The river, the List of longest rivers of the United States (by main stem), 5th longest in the United St ...

and divert it to Colorado's

Front Range The Front Range is a mountain range of the Southern Rocky Mountains of North America located in the central portion of the U.S. State of Colorado, and southeastern portion of the U.S. State of Wyoming. It is the first mountain range encounter ...

and plains. In Colorado, approximately 80% of the state's precipitation falls on the West Slope, in the Rocky Mountains, while around 80% of the state's growing population lives along the eastern slope, between the cities of

Fort Collins A fortification (also called a fort, fortress, fastness, or stronghold) is a military construction designed for the defense of territories in warfare, and is used to establish rule in a region during peacetime. The term is derived from La ...

and

Pueblo Pueblo refers to the settlements of the Pueblo peoples, Native American tribes in the Southwestern United States, currently in New Mexico, Arizona, and Texas. The permanent communities, including some of the oldest continually occupied settlement ...

. Eleven reservoirs, about 18 dams and dikes, the Alva B. Adams Tunnel under the Continental Divide, as well as six power plants, make up the project. The C-BT is owned and primarily managed by the U.S. Bureau of Reclamation's Eastern Colorado Area Office under its Great Plains Region.

History

The project was built, is owned, and is primarily operated by the federal Bureau of Reclamation under 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 ...

. By the late 1890s, farmers in northeastern Colorado realized water rights in the area had become over-appropriated. In order to survive the agricultural season, additional water supplies would be needed. Prior to the

Dust Bowl The Dust Bowl was a period of severe dust storms that greatly damaged the ecology and agriculture of the American and Canadian prairies during the 1930s. The phenomenon was caused by a combination of natural factors (severe drought) and hum ...

era, agriculture in this section of the state had relied upon sources such as Boulder Creek, St. Vrain Creek,

Little Thompson River The Little Thompson River is a tributary of the Big Thompson River and thence the South Platte River in the U.S. state of Colorado. The river's headwaters lie in the Roosevelt National Forest. It flows east through the foothills of the Rocky M ...

Big Thompson River The Big Thompson River is a tributary of the South Platte River, approximately long, in the U.S. state of Colorado. Originating in Forest Canyon in Rocky Mountain National Park, the river flows into Lake Estes in the town of Estes Park and ...

and the

Cache La Poudre River The Cache la Poudre River ( ), also known as the Poudre River, is a river in the state of Colorado in the United States. Name The name, , is a corruption of the original , or " cache of powder". It refers to an incident in the 1820s when F ...

, all of which are a part of the South Platte River basin and flow into the

South Platte River The South Platte River is one of the two principal tributaries of the Platte River. Flowing through the U.S. states of Colorado and Nebraska, it is itself a major river of the American Midwestern United States, Midwest and the American Sou ...

before the South Platte reaches Greeley, Colorado. In search of a solution, farmers and their representatives approached the Bureau of Reclamation. In the late 1930s a solution was found: divert the water via a -long tunnel under 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 ...

and

Rocky Mountain National Park Rocky Mountain National Park is a List of national parks of the United States, national park of the United States located approximately northwest of Denver in north-central Colorado, within the Front Range of the Rocky Mountains. The park is s ...

. (Tunnel West Portal , Tunnel East Portal ) The proposed water diversion was extensive and the project could not have been constructed without compensation to the West Slope for the water sent east. As a result, the first feature built on the C-BT was Green Mountain Dam and Reservoir, a West Slope facility designed to provide for future water demands in the state's Upper Colorado River Basin. The project was authorized by President

Franklin Delano Roosevelt Franklin Delano Roosevelt (January 30, 1882April 12, 1945), also known as FDR, was the 32nd president of the United States, serving from 1933 until his death in 1945. He is the longest-serving U.S. president, and the only one to have served ...

in 1937. Construction began on Green Mountain in the northern part of Summit County in 1938. Construction on the project continued through most of the next 20 years.

Benefits

While the project was originally built for agricultural purposes, it serves multiple demands including municipal and industrial supply, hydro-power generation, recreation, and fish and wildlife. In recent years, however, water supply demands have shifted making municipal and industrial supply the main water beneficiary, rather than irrigation. Today, the "C-BT" serves over 33 cities and towns in northeastern Colorado, including

, Greeley, Loveland,

Estes Park Estes Park () is a List of municipalities in Colorado#Statutory town, statutory town in Larimer County, Colorado, United States. The town population was 5,904 at the 2020 United States census. Estes Park is a part of the Fort Collins, CO Metropo ...

Boulder In geology, a boulder (or rarely bowlder) is a rock fragment with size greater than in diameter. Smaller pieces are called cobbles and pebbles. While a boulder may be small enough to move or roll manually, others are extremely massive. In ...

, and Sterling, encompassed by 7 counties, providing a secondary source of water for around 1 million people and an irrigated area of 640,000 acres (2,500 km²). Although water rights allow for up to of water a year to be diverted, annual diversions average around , instead. A drop of over 2000 vertical feet from the Rockies down to the plains allows for power generation. Seven power plants on the project produce an average supply of 759 million kilowatt hours of electricity a year. Like the water supply, generated electricity is supplemental. Electricity produced on the C-BT is a source of "peaking power" and is marketed by the

Department of Energy A ministry of energy or department of energy is a government department in some countries that typically oversees the production of fuel and electricity; in the United States, however, it manages nuclear weapons development and conducts energy-rela ...

via its

Western Area Power Administration As one of the four power marketing administrations within the U.S. Department of Energy, the Western Area Power Administration (WAPA)'s role is to market wholesale hydropower generated at 57 hydroelectric federal dams operated by the Bureau of Re ...

Project features

West-slope collection system

An extensive series of reservoirs, pumps and conduits on the west side of the Rockies serve to collect water from the headwaters of the Colorado River, as well as two tributaries, Cottonwood Creek and the

Fraser River The Fraser River () is the longest river within British Columbia, Canada, rising at Fraser Pass near Blackrock Mountain (Canada), Blackrock Mountain in the Rocky Mountains and flowing for , into the Strait of Georgia just south of the City of V ...

Lake Granby Lake Granby is the third largest body of water in Colorado. It was created by the erection of Granby Dam, completed in 1950, as part of the Bureau of Reclamation's Colorado-Big Thompson Project. Water from Lake Granby is pumped via the Farr Pum ...

, located in eastern Grand County, is the primary C-BT storage facility, with a capacity of . The reservoir is held by the high

Granby Dam Granby Dam (National ID # CO01656) is an earthfill dam that dams the Colorado River northeast of Granby, Colorado in Grand County, Colorado Grand County is a county located in the U.S. state of Colorado. As of the 2020 census, the pop ...

and of auxiliary dikes. Willow Creek Reservoir is built on Willow Creek, which is located west of Lake Granby, and provides a source from which water is diverted and pumped to Granby. Windy Gap Reservoir is a small diversion facility located directly below the confluence of the Colorado and Fraser rivers, about downstream of Granby. Water from the Fraser River, as well as other inflows to the Colorado below Granby Dam, is diverted here and pumped eastwards to Lake Granby. The Windy Gap project is not owned by the Bureau of Reclamation, but by the Municipal Subdistrict, a consortium of 14 Front Range cities, water providers and an electric utility. However, Windy Gap water uses the storage and distribution facilities of the Bureau of Reclamation's C-BT. From Lake Granby the water is lifted up to Shadow Mountain Lake, which is located on the Colorado River west of the natural Grand Lake. The two bodies of water are connected by a short channel which allows water to flow freely to the intake of the Alva B. Adams Tunnel on Grand Lake's eastern shore. The water then flows under the Continental Divide through the Adams Tunnel, which can carry up to to the eastern slope.

East-slope hydroelectric system

Once the water emerges from the Adams Tunnel just southwest of Estes Park, the system is almost entirely gravity powered, dropping some as it descends to the foothills of the Rocky Mountains west of Loveland. The tunnel outlet is located at East Portal Reservoir, a small regulating pool on the Wind River. From here it is transported via an inverted

siphon A siphon (; also spelled syphon) is any of a wide variety of devices that involve the flow of liquids through tubes. In a narrower sense, the word refers particularly to a tube in an inverted "U" shape, which causes a liquid to flow upward, abo ...

across the Aspen Creek valley and drops to Mary’s Lake, where it drives the 8.1

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) Mary’s Lake Powerplant. Mary’s Lake is a small natural lake enlarged to form a second regulatory reservoir. The water then drops to the 45-MW Estes Powerplant at

Lake Estes Lake Estes is a reservoir in Estes Park, Colorado created by Olympus Dam. The lake has a shoreline of about and a surface area of . The reservoir lies on the Big Thompson River and is a component of the Colorado-Big Thompson Project. Lake Est ...

, which is formed by

Olympus Dam Olympus Dam is a dam located on the Big Thompson River, in the town of Estes Park, Colorado. The reservoir behind the dam, Lake Estes, is the main source of drinking water for Estes Park. The dam was constructed between 1947 and 1949. The dam i ...

on the Big Thompson River at Estes Park. With its small storage capacity, Lake Estes provides limited regulation of both C-BT water and the natural flows of the Big Thompson River, including diversion of some Big Thompson River water for non-consumptive use in hydroelectric power generation. From Lake Estes the water travels east via the Olympus and Pole Hill Tunnels to the 38.2 MW Pole Hill Powerplant, where it drops , and flows via the shorter Rattlesnake Tunnel to Pinewood Lake. The water then enters the Bald Mountain Tunnel, heading east to a final drop of at 94.5 MW Flatiron Powerplant. From the tailrace of the powerplant the water enters Flatiron Reservoir from which the water is distributed to the eastern slope, including returning native hydropower water to the river at the mouth of the Big Thompson Canyon.

Foothill distribution facilities

Once the water reaches Flatiron Reservoir, it splits into two branches which distribute water to about of the Front Range Corridor, from

to near

Denver Denver ( ) is a List of municipalities in Colorado#Consolidated city and county, consolidated city and county, the List of capitals in the United States, capital and List of municipalities in Colorado, most populous city of the U.S. state of ...

. The northern branch consists of the Horsetooth Feeder Canal and tunnels which feeds water by gravity to

Horsetooth Reservoir Horsetooth Reservoir (often known locally as Horsetooth) is a large reservoir in southern Larimer County, Colorado, in the foothills just west of the city of Fort Collins, Colorado. The reservoir runs north-south for approximately 6.5 miles (10 ...

. The reservoir is formed by four dams in the hills west of Fort Collins and has a total capacity of . The northern end of the reservoir outlets into the Charles Hansen Supply Canal, which mainly supplies agriculture in the

Cache la Poudre River The Cache la Poudre River ( ), also known as the Poudre River, is a river in the state of Colorado in the United States. Name The name, , is a corruption of the original , or " cache of powder". It refers to an incident in the 1820s when F ...

valley. There is a smaller outlet at Soldier Canyon which provides water to the Fort Collins area. Water flowing into the southern branch must be pumped into Carter Lake Reservoir, located west of Berthoud. The reservoir can hold up to of water. During times of peak power demand, water can be released back from Carter into Flatiron via a pump-generating unit. Water flows south from Carter Lake into the St. Vrain Supply Canal, which provides water to the

and Saint Vrain Creek. From the end of the St. Vrain Canal the Boulder Creek Supply Canal extends southward to Boulder Creek, and the South Platte Supply Canal extends northeast from there to the

References

External links

The Colorado–Big Thompson Project
US Bureau of Reclamation
four-volume C-BT 'blue books'
US Bureau of Reclamation. (1957). Colorado-Big Thompson project, constructed 1938-56: technical record of design and construction. Denver: United States G.P.O..
A River Routed Under the Mountains
June 9, 2022,

NASA Earth Observatory NASA Earth Observatory is an online publishing outlet for NASA which was created in 1999. It is the principal source of satellite imagery and other scientific information about the climate and the environment which are being provided by NASA for ...

, features satellite photos of many features of the project.
The Great Plains Region
US Bureau of Reclamation
The Eastern Colorado Area Office
US Bureau of Reclamation

US Bureau of Reclamation
'' "Man-made River Pierces The Rockies" '', September 1947, Popular ScienceU.S. Department of the Interior: Water and Power Resources Service, Project Data, pps 251-292. United States Government Printing Office, 1981.
{{DEFAULTSORT:Colorado-Big Thompson Project Buildings and structures in Colorado Colorado River Energy infrastructure in Colorado Interbasin transfer United States Bureau of Reclamation Water supply and sanitation in the United States Hydroelectric power plants in Colorado