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Colorado-Big Thompson Project
The three lakes collection system, seen from the air, surrounded by beetle killed pine.
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Colorado-Big Thompson Project (85 KB) (pdf)
General Description | Plan | Development | Benefits

General Description

The project spreads over approximately 250 miles in the State of Colorado. It stores, regulates, and diverts water from the Colorado River on the western slope of the Continental Divide to the eastern slope of the Rocky Mountains. It provides supplemental water for irrigation of about 720,000 acres of land, municipal and industrial use, hydroelectric power, and water-oriented recreation opportunities.

Major features of the project include dams, dikes, reservoirs, powerplants, pumping plants, pipelines, tunnels, transmission lines, substations, and other associated structures.

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The project diverts approximately 260,000 acre-feet of water annually (310,000 acre-feet maximum) from the Colorado River headwaters on the western slope to the Big Thompson River, a South Platte River tributary on the eastern slope, for distribution to project lands and communities. The Northern Colorado Water Conservancy District apportions the water used for irrigation to more than 120 ditches and 60 reservoirs. Eleven communities receive municipal and industrial water from the project. Electric power produced by six powerplants is marketed by the Western Division of the Pick-Sloan Missouri Basin Program.

The western slope collection system traps runoff from the high mountains and stores, regulates, and conveys the water to the Alva B. Adams Tunnel for diversion under the Continental Divide.

To assure irrigation and power generation under prior rights on the Colorado River, Green Mountain Reservoir was constructed on the Blue River. Spring runoff is stored in this reservoir and later released to meet the requirements of the Colorado River, and to allow diversion of water by the project throughout the year.

Irrigation systems on the Colorado River, above the Blue River confluence, were improved to enable continued use of existing rights. Releases are made from Lake Granby to maintain the Colorado River as a fine fishing stream.

The principal storage features are Lake Granby and Granby Dam, located on the Colorado River near Granby. Willow Creek, a tributary below Lake Granby, is diverted by Willow Creek Dam and Canal. Willow Creek Pumping Plant lifts the water 175 feet; it then flows by gravity to Lake Granby.

Granby Pumping Plant lifts the water 125 feet from Lake Granby to Granby Pump Canal. The canal conveys the water 1.8 miles to Shadow Mountain Lake, which also intercepts North Fork flows of the Colorado River. Shadow Mountain Lake connects with Grand Lake to make a single body of water from which diversions flow to the Alva B. Adams Tunnel to begin the journey to the eastern slope.

Emerging from Alva B. Adams Tunnel into the East Portal Reservoir, the water flows across Aspen Creek Valley in a siphon and then under Rams Horn Mountain through a tunnel. At this point, it enters a steel penstock and falls 205 feet to Marys Lake Powerplant. This powerplant is located on the west shore of Marys Lake, which provides afterbay and forebay capacity for reregulating the flow. Between Marys Lake and Estes Powerplant, on the shore of Lake Estes, the water is conveyed by Prospect Mountain Conduit and Prospect Mountain Tunnel.

Lake Estes, below Estes Powerplant, is formed by Olympus Dam constructed across the Big Thompson River. The afterbay storage in Lake Estes and the forebay storage in Marys Lake enable the Estes Powerplant to meet daily variations in energy demand.

Water from Lake Estes and some Big Thompson River floodwaters are conveyed by Olympus Siphon and Tunnel and Pole Hill Tunnel and Canal to a penstock through which the water drops 815 feet to Pole Hill Powerplant. It is then routed through Pole Hill Powerplant Afterbay, Rattlesnake Tunnel, Pinewood Lake, and Bald Mountain Pressure Tunnel, and dropped 1,055 feet through two penstocks to Flatiron Powerplant. This powerplant discharges into Flatiron Reservoir, which regulates the water for release to the foothills storage and distribution system. The afterbay storage in Flatiron Reservoir and the forebay storage in Pinewood Lake enable Flatiron Powerplant to meet daily power loads.

Southward, the Flatiron reversible pump lifts water from Flatiron Reservoir, a maximum of 297 feet and delivers it through Carter Lake Pressure Conduit and Tunnel to Carter Lake. When the flow is reversed, the unit acts as a turbine-generator and produces electric energy.

The St. Vrain Supply Canal delivers water from Carter Lake to the Little Thompson River, St. Vrain Creek, and the Boulder Creek Supply Canal. The latter delivers water to Boulder Creek and Boulder Reservoir. The South Platte Supply Canal, diverting from Boulder Creek, delivers water to the South Platte River.

Northward, the Charles Hansen Feeder Canal transports water from Flatiron Reservoir to the Big Thompson River and Horsetooth Reservoir. The canal crosses the Big Thompson River in a siphon above the river and highway. Water from the Big Thompson River can be diverted into the canal by Tunnel No.1, Horsetooth Supply Conduit.

Project water deliveries and Big Thompson River water to be returned to the river are dropped through a chute from the feeder canal ahead of the siphon crossing, or are passed through the Big Thompson Powerplant to convert the available head to electric energy.

Horsetooth Reservoir is west of Fort Collins between two hogback ridges, where Horsetooth Dam closes the gap at one end. Soldier, Dixon, and Spring Canyon Dams and Satanka Dike close the remaining gaps.

An outlet at Soldier Canyon Dam supplies water to Fort Collins, rural water districts, Colorado State University, and the Dixon Feeder Canal for the irrigated area cut off from its water supply by the reservoir.

The principal outlet from Horsetooth Reservoir is through Horsetooth Dam into the Charles Hansen Canal. This canal delivers water to a chute discharging into the Cache la Poudre River and to a siphon crossing the river to supply the Poudre Valley and Reservoir Company Canal. A turnout supplies the Greeley municipal water works. Water is delivered to the river to replace, by exchange, that water diverted upstream of the North Poudre Supply Canal, which conveys it to the North Poudre Ditch.

Facility Descriptions
Green Mountain Dam, Reservoir, and Powerplant

Green Mountain Dam is on the western slope 13 miles southeast of Kremmling on the Blue River, a tributary of the Colorado. This dam provides replacement storage for water diverted by the project to the eastern slope. The dam is an earthfill structure, 309 feet high, with a crest length of 1,150 feet and a volume of 4,360,211 cubic yards. The reservoir has a total capacity of 153,639 acre-feet. The powerplant has two units with a total installed generating capacity of 21,600 kilowatts.

Granby Dam and Lake Granby

Granby Dam is located on the Colorado River about 5.5 miles northeast of Granby. It collects and stores most of the project water supply, including the flow of the Colorado River and water pumped from Willow Creek. The dam is constructed of compacted earthfill, 298 feet high, with a crest length of 861 feet. There are 12,722 feet of auxiliary dikes. The reservoir has a capacity of 539,800 acre-feet. Total volume of the dam is 2,974,000 cubic yards. The dikes have a total volume of 1,739,000 cubic yards.

Willow Creek Dam, Reservoir, and Pumping Plant

Willow Creek Dam is 127 feet high, 1,100 feet long, and constructed of earthfill. There are 3.4 miles of canals with a capacity of 400 cubic feet per second and a pumping plant with two 200-cubic-foot-per-second pumps that lift water 175 feet into Lake Granby. The dam diverts an average of 40,000 acre-feet of water each year from Willow Creek into Lake Granby. The reservoir capacity is 10,600 acre-feet.

Granby Pumping Plant and Pump Canal

Water is pumped from Lake Granby into Shadow Mountain Lake by Granby Pumping Plant and Canal. The pumping plant contains three centrifugal pumps with a total capacity of 600 cubic feet per second at 186-foot head. The pumping lift ranges from 85 to 186 feet according to the water surface elevation in Lake Granby. The water is discharged into a canal which has a capacity of 1,100 cubic feet per second, and conveyed 1.8 miles to Shadow Mountain Lake.

Shadow Mountain Dam and Reservoir

Shadow Mountain Dam, located on the Colorado River below its confluence with the Grand Lake outlet, is an earthfill structure 63 feet high and 3,077 feet long. The reservoir formed by the dam has a total capacity of 18,400 acre-feet and is linked to Grand Lake through a connecting channel. Shadow Mountain Lake receives the water pumped from Lake Granby and also intercepts North Fork flows of the Colorado River. Project water is released from Grand Lake directly into the Alva B. Adams Tunnel, through which it flows to the eastern slope of the Continental Divide.

Alva B. Adams Tunnel

This 9.75-foot-diameter, 13-mile-long tunnel extends from Grand Lake through the Continental Divide to a point 4.5 miles southwest of Estes Park. It has a capacity of 550 cubic feet per second.

East Slope Power System-Upper

The structures of this system convey water 4.3 miles from the east portal of Alva B. Adams Tunnel to the Big Thompson River.

Emerging from the tunnel into the East Portal Reservoir, the water flows across Aspen Creek Valley in a siphon and then under Rams Horn Mountain in a tunnel. At this point, the water enters a steel penstock and falls 205 feet to Marys Lake Powerplant, which has an installed capacity of 8,100 kilowatts. This plant is located on the west shore of Marys Lake, which has been enlarged by diking the small natural basin to provide afterbay and forebay capacity for reregulating the flow. From Marys Lake to Estes Powerplant, the water is dropped 482 feet in a pressure system consisting of Prospect Mountain Conduit and Prospect Mountain Tunnel.

Estes Powerplant contains three generating units served by three 78-inch-diameter penstocks about 0.75 mile long. The installed plant capacity is 45,000 kilowatts when operating under an average net head of 482 feet.

Olympus Dam, a zoned earthfill structure with a concrete overflow spillway, is 70 feet high and has a crest length of 1,951 feet. It impounds Lake Estes on the Big Thompson River and provides regulating capacity for energy purposes. The lake has a total capacity of about 3,100 acre-feet and controls the discharges from Estes Powerplant, river inflow and outflow, and releases of project water to the Lower East Slope Power System.

East Slope Power System-Lower

This system conveys project water from Lake Estes in a southeasterly direction to the Foothills storage and supply system. Project water released from Lake Estes flows through Olympus Siphon and Tunnel and Pole Hill Tunnel and Canal into Pole Hill Penstock and Powerplant. Water also can be released from Lake Estes to the Big Thompson River. Leaving Pole Hill Powerplant Afterbay, the water enters Rattlesnake Tunnel and flows into Pinewood Lake formed by Rattlesnake Dam. Bald Mountain Tunnel carries the water into the Flatiron Penstocks and Powerplant which discharges into Flatiron Reservoir, where it is stored for irrigation use. Pole Hill Powerplant operates under an average net head of 815 feet with a generating capacity of 33,250 kilowatts.

The Flatiron Powerplant operates under an average net head of 1,055 feet, with a generating capacity of 71,500 kilowatts. The powerplant contains two main power units and a reversible 13,000-horsepower pump-turbine unit which lifts water southward from Flatiron Reservoir to Carter Lake. This unit is capable of discharging a maximum of 370 cubic feet per second into Carter Lake and normally operates on surplus or off-peak power generated by other power units of the project system.

The pumping unit at Flatiron Powerplant pumps from Flatiron Reservoir to Carter Lake through a 1.4-mile-long connecting pressure tunnel. The pumping lift through this tunnel ranges from 200 to 300 feet, depending on the water surface elevation in Carter Lake. During peak load demands on the project system, water can be released from Carter Lake to flow back into Flatiron Reservoir, and at such times the pump-turbine operates in reverse to generate 8,500 kilowatts of power.

Flatiron Dam provides afterbay storage for water discharged from the powerplant. The water then flows by gravity northward through the Charles Hansen Feeder Canal, to and across the Big Thompson River, and on to Horsetooth Reservoir for delivery to the Poudre River, Poudre Valley Canal, and, by exchange, to the North Poudre Supply Canal.

Water pumped southward into Carter Lake is stored for irrigation deliveries to the Little Thompson River, St. Vrain Creek, Boulder Creek, and the South Platte River.

Carter Lake Dam and Reservoir

Carter Lake is one of the two main project storage reservoirs in the East Slope distribution system. Water is stored in this reservoir for delivery to the Little Thompson River, St. Vrain Creek, Boulder Creek, and the South Platte River, for return to Flatiron Reservoir for use in the Big Thompson or Cache la Poudre Valleys, or for power generation.

Carter Lake Reservoir is formed in a natural basin in the foothills by a 214-foot-high earthfill dam and two smaller dams across low saddles in the surrounding hills. The reservoir has a total capacity of 112,230 acre-feet.

St. Vrain Supply Canal

Leading from the Carter Lake outlet, the St. Vrain Supply Canal extends southward 9.8 miles to St. Vrain Creek near Lyons. It consists of an open canal, siphons, tunnels, drops, and flumes designed to convey 625 cubic feet per second of water to the Little Thompson River turnout and 575 cubic feet per second from the turnout to St. Vrain Creek.

Boulder Creek Supply Canal

Boulder Creek Supply Canal begins at the turnout near the end of the St. Vrain Supply Canal, crosses St. Vrain Creek by a siphon, and extends southeasterly 15.7 miles It discharges into Boulder Creek about 6 miles east of Boulder. The canal has a carrying capacity of 200 cubic feet per second.

Near the lower end of the canal, the city of Boulder constructed Boulder Reservoir to be used for storage and regulation of the city`s water for replacement water carried in the canal. This reservoir was built under an agreement between the city and the Northern Colorado Water Conservancy District. Under the agreement, the reservoir provides 175 cubic feet per second of flow for the South Platte Supply Canal.

South Platte Supply Canal

This canal extends from Boulder Creek generally north-east to the South Platte River, a distance of about 32.2 miles. The capacity of the canal is 230 cubic feet per second at the start and progressively decreases. Near the lower end of the canal, the Platte Valley Irrigation Co. constructed Coal Ridge Waste Lake for storage. This reservoir was built under an agreement with the Northern Colorado Water Conservancy District. Under the agreement, the lake provides 100 cubic feet per second of South Platte Supply Canal flows.

Charles Hansen Feeder Canal

Beginning at the outlet of Flatiron Reservoir, the Charles Hansen Feeder Canal extends northward to Horsetooth Reservoir. The canal has a capacity of 930 cubic feet per second to the Big Thompson River and 550 cubic feet per second to the reservoir. The canal crosses the Big Thompson River and U.S. Highway 34 in a 9-foot-diameter steel siphon. A control structure ahead of the Big Thompson River Siphon provides a means to release irrigation water to the Big Thompson River to bypass surplus water, and to release water to the Big Thompson Powerplant. The Horsetooth Supply Conduit, an important feature of the canal, diverts water from the Big Thompson River about 1 mile upstream from the control structure and delivers it via a tunnel to the Charles Hansen Feeder Canal above the control structure. Diverted water is used for power generation at the Big Thompson Powerplant, or water surplus to the needs of the Big Thompson Valley can be stored in Horsetooth Reservoir. North of the Big Thompson River, the canal passes through four concrete-lined tunnels; the outlet of the last tunnel discharges the water into the Horsetooth Reservoir.

Big Thompson Powerplant

The Big Thompson Powerplant is on the Big Thompson River about 9 miles west of Loveland and just downstream from the river crossing of the Charles Hansen Feeder Canal. The plant operates under an effective head of 180 feet and has a generating capacity of 4,500 kilowatts.

Horsetooth Reservoir

Horsetooth Reservoir, with a total capacity of about 151,750 acre-feet, furnishes the main supply for the Poudre Valley, where 50 percent of the project water is used. The reservoir is 6.5 miles long, and is formed by four large earthfill dams. Horsetooth Dam closes the northern end of the valley, and Soldier Canyon, Dixon Canyon, and Spring Canyon Dams close natural outlets eroded through the hogback ridge. These dams have heights of 155, 226, 240, and 220 feet, respectively. The dams contain more than 10 million cubic yards of earthfill.

Charles Hansen and North Poudre Supply Canals

Outlets at Horsetooth Dam discharge into the Charles Hansen Canal, which is designed to carry a maximum of 1,500 cubic feet per second northward 5.1 miles to the Cache la Poudre River. Project water released into the river at this point is used to supplement the water supply of irrigation systems stemming from the river. It also serves as replenishment for the water taken from the river a few miles upstream by the North Poudre Supply Canal, a 12.5-mile-long canal which carries supplemental water to the North Poudre Ditch. The 0.5-mile, 250-cubic-foot-per-second Windsor Extension Canal takes part of the Poudre supply across the river to the Poudre Valley Canal, an older waterway that serves a portion of the conservancy district.

The Soldier Canyon Dam outlet supplies water to Colorado State University, to the small Dixon Feeder Canal for the irrigated area cut off from its water supply by Horsetooth Reservoir, to Fort Collins, and to rural water districts.

The Cache la Poudre, Big Thompson, and Little Thompson Rivers, and St. Vrain and Boulder Creeks are tributaries of the South Platte River, through which water imported from the western slope is supplied to the South Platte River Basin system. This supplemental water is used to alleviate the critical shortages that have hampered and restricted the cultivation of fertile lands in the South Platte River Valley.

Power Distribution System

Power transmission facilities include nearly 677 miles of transmission lines, 35 permanent substations, 2 mobile substations, 1 mobile transformer, 22 metering stations, and 6 permanent service shops. With the exception of 3 miles of steel tower construction and 13.1 miles of submarine-type conduit, the transmission circuits are of wood pole H-frame construction. The submarine-type conduit is the connection between eastern and western slope circuits and is in a nitrogen gas-filled pipe suspended from the top of the Alva B. Adams Tunnel. Project power facilities are interconnected with plants of the North Platte, Kendrick, Riverton, and Shoshone Projects, and are tied into the lines of the Public Service Company of Colorado at five different locations in Colorado. Most of these power features were transferred to the Department of Energy`s (DOE) Western Area Power Administration upon the creation of DOE in 1977.

Operating Agencies

The Bureau of Reclamation operates all project features on the western slope, including power, storage, and carriage, and all similar works on the eastern slope above the supply canals leading from Carter Lake and Horsetooth Reservoirs. All project works below these two reservoirs are operated and maintained by the Northern Colorado Water Conservancy District.

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In 1870, before statehood was achieved by the Colorado Territory, the Union Colony of 2,000 people was established at Greeley. This marked the inception of cooperative irrigation in the South Platte River Valley and the beginning of an era in which irrigation became important in the economic development of northeastern Colorado.

The Union Colony started with construction of ditches to supply direct flow from the river to 12,000 acres. The venture was so successful that by 1900 the streams were over appropriated and attention was given to developing of plains reservoirs to store the spring floods. By 1910, most of the better reservoir sites were used and few of possibilities were apparent, except costly transmountain diversion.

During these years, the increasing demand for agricultural products for a growing population and the tendency to prepare as large an irrigation system as possible to spread the cost of the works, resulted in over-expansion, especially in years of high and adequate runoff. Subnormal or even normal runoff years were critical for much of the area so developed. Water shortages continually plagued the irrigators.


The idea of transmountain water diversions had been in existence since 1889, when the Colorado legislature appropriated money to investigate such a proposal. Progressive steps in legislation finally led, in 1922, to the signing of the Colorado River Compact, which apportioned the Colorado River water between the upper and lower basin States. Later, the Boulder Canyon Act provided funds for determining the amount of lands that were or could be irrigated in the Colorado River Basin. A plan was developed whereby Colorado River water could be diverted into watersheds in northeastern Colorado where there was a surplus of irrigable lands and a shortage of water. The upper basin States successfully developed a compact in 1948 prorating the upper basin's share based on the 1922 compact.

Engineering investigations of the Colorado-Big Thompson Project began in 1933, when a preliminary survey to determine the feasibility of a project was undertaken. A favorable report was presented in 1934. In January 1935, the Bureau of Reclamation was allotted funds by the Public Works Administration to make a new study.

Project construction was contingent upon the formation of a conservancy district to contract with the United States Government. Accordingly, the Colorado Water Conservancy Law was passed by the Colorado legislature in 1937. The law contains several unique features. One provides that a conservancy district May be organized by any district court upon petition of a stipulated number of property owners; another recognizes that all who benefit as a result of project development should contribute to its cost and operation in proportion to those benefits.

The Northern Colorado Water Conservancy District was organized in 1937 with boundaries which include large areas of Larimer, Boulder, and Weld Counties, and portions of Morgan, Washington, Logan, and Sedgwick Counties.


First construction funds were provided in the Interior Department Appropriation Act of August 9, 1937 (50 Stat. 595). The Secretary`s finding of feasibility was approved by the President on December 21, 1937.


Construction of the project began at Green Mountain Dam during November 1938. The first power was generated at the Green Mountain Powerplant in May 1943; all construction of the dam and powerplant was completed in October 1943. Construction of Granby Dam started in 1941, and of Alva B. Adams Tunnel in the summer of 1940. Work was curtailed during World War II, but not entirely stopped. At the end of the war, the tempo of construction was speeded up. During 1956, all major features were essentially completed except the Big Thompson Powerplant, which was completed in 1959.

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The Colorado-Big Thompson Project helps stabilize the agricultural and industrial economy of northeastern Colorado. It is particularly effective each year during late summer months of the irrigation season, and has a tremendous impact throughout the season in drought years.

Principal crops include sugar beets, potatoes, beans, corn, small grains, fruits, alfalfa, vegetables, dairy products, poultry, and eggs. In addition, lambs, hogs, and cattle are fattened from the byproducts of the sugar beets.

Municipal and Industrial

Municipal supplies have been an important aspect in the distribution of project water. Originally, nine communities had allotments totaling 44,950 acre-feet. Eleven communities now receive full or supplemental supplies. Each year, as urban population increases, irrigation allotments are transferred to domestic purposes. The dependable availability of water continues to attract a variety of industries.


About two million people visit the man-made lakes annually to enjoy fishing, motor and sail boating, water skiing, swimming, camping, hiking, and picnicking. Trout, kokanee, bass, walleye, and perch are the principal fish caught in the clear, cool waters. Ice fishing and snowmobiling have become favorite winter sports.

For specific information about any of these recreation sites, click on the name below.

Carter Lake
East Portal Reservoir
Flatiron Reservoir
Green Mountain Reservoir
Horsetooth Reservoir
Lake Estes
Lake Granby
Marys Lake
Pinewood Lake
Rocky Mountain National Park
Shadow Mountain Lake
Willow Creek Reservoir

Hydroelectric Power

From the eastern portal of the Alva B. Adams Tunnel, water descends about 2,800 feet to the foothills. Nearly every foot of the head is used for hydroelectric power generation. Gross generation averages 760 million kilowatt-hours of which 70 million kilowatt-hours are used by project pumps and 690 million kilowatt-hours are marketed to customers in northern Colorado, eastern Wyoming, and western Nebraska. The power produced at the Bureau powerplants is marketed by DOE.

The water and power control center for Reclamation's reservoirs and powerplants in Wyoming, Colorado, and western Nebraska is located in Mills, Wyo. This Western Division of the Missouri River Basin is an interconnected system of 19 Reclamation powerplants and 832,780 kilowatts of installed capacity.

Flood Control

The Colorado Big Thompson Project has provided an accumulated $316,000 in flood control benefits from 1950 to 1999.

Colorado River Basin    
Green Mountain 88 0
Lake Granby 218 0
Shadow Mountain Lake 10 0
Willow Creek 0 0
South Platte River Basin 0 0
Marys Lake 0 0
Lake Estes 0 0
Pinewood Lake 0 0
Flatiron Reservoir 0 0
Carter Lake 0 0
Horsetooth Reservoir 0 0

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Last updated: Jul 21, 2016