Chimney Hollow Reservoir, the main storage reservoir for the Windy Gap Project in Colorado, US, was completed in 2026 to store and bring water to nine municipal utilities, two water districts and one electric utility. At a project price of about US$750m, the reservoir will meet the needs of hundreds of thousands of municipal water users while providing recreation opportunities to those who live throughout the region. While substantial construction was finished in 2026, the project is one that took decades to reach its conclusion.
Early days
The first discussion of using water from the Fraser River for northeastern Colorado came via the Colorado-Big Thompson (C-BT) Project, which was authorised in 1937 and built over the course of two decades. This scheme harnesses water from several tributaries of the Colorado River, stores it in Lake Granby and transports it via the Alva B. Adams Tunnel to more than 1.1 million users on the east side of the Continental Divide. As that project was being built, however, design changes occurred that removed some of the water-supplying creeks from the final project. When community leaders recognised the success the Colorado-Big Thompson Project was having on their communities, they realised the success could have an unintended effect of removing water from agricultural production in project delivery boundaries. Thus, they looked again at the original C-BT Project designs and filed for a water right below the confluence of the Colorado and Fraser rivers to capture water that was not being used by any entities on the Western Slope nor was included in the original project.
This water right, in a place called Windy Gap, would be used by those communities in a project separate from the C-BT Project and not managed by the federal government. Through the 1970s, those communities created a governance structure called the Municipal Subdistrict, and received the permits and contracts needed to build the Windy Gap Project, consisting of a small reservoir on the Colorado River, a pump plant and delivery pipelines from Windy Gap Reservoir to Lake Granby. Windy Gap Reservoir opened in 1985, and it delivers water to Lake Granby for those years when its junior priority has access to water and there is space in Lake Granby to store it. In some years, the junior water rights are never in priority, and more concerning, in other years Lake Granby is full of C-BT Project water and cannot store the external water from Windy Gap. By contract, the non-federal water is the first to be released from Lake Granby when water levels reach the spillway.
Drought
In 2002, a major drought in Colorado put these issues on the front burner. Not only was there no water to divert that year, but water from previous years was not available because it was unable to be stored in Lake Granby. At that point, 12 users of Windy Gap Project water came together to start the permitting process to build storage for that water. While several locations were evaluated to build a reservoir, one emerged that would require relatively little environmental work or displacement of current residents: the Chimney Hollow valley.
Owned by the computer firm Hewlett-Packard Inc., now known as HP Inc., the valley was to be an employee retreat centre for the company’s locations in Loveland, Greeley and Fort Collins. By 2004, however, the need for such a site had declined, and the company was looking to sell the property. Unfortunately, the Municipal Subdistrict could not afford the property; Larimer County, whose voters had approved a sales tax to purchase property for parks and open spaces, also could not afford the sales price. The two learned of each other’s interest in the property and pooled their funds. That amount, plus a state grant, enabled the purchase of the property under joint ownership, with the Municipal Subdistrict having rights to the land below the high-water mark for the reservoir plus other water-focused property, and Larimer County owning the acreage above the reservoir for future recreation activities.
Permitting for the project included working with Grand County for approvals to divert water from the Colorado River more consistently than previously. From that work came an agreement to build a new channel to divert water around the on-stream Windy Gap Reservoir. This new waterway, called the Colorado River Connectivity Channel, was completed in 2024, and in future years will provide additional public-access fishing locations. Federal permits were issued for the project in 2017.
In late 2017, a group of environmental protection nonprofit organisations filed a lawsuit against the federal agencies who issued those permits. That lawsuit was successfully defeated at the federal district court in Denver, and the federal appeals court process concluded with a settlement between the Municipal Subdistrict and the project’s opponents to fund US$15m of additional work in the Colorado River headwaters for improved river health and recreation.
Construction begins
The Chimney Hollow reservoir was considered to be the best site option. It’s adjacent to the C-BT project facilities, making it easy to get water in and out of the reservoir and back into the federal system for delivery.
Construction began on Chimney Hollow Reservoir in August 2021 and continued for more than four years. The project’s design engineer was Stantec, based in Edmonton, Canada, and the construction manager was Black & Veatch, based in Overland Park, Kansas. Because the Chimney Hollow valley is a geologic valley, created by the uplift of the Rocky Mountains, no year-round stream is impacted by the reservoir’s creation. However, the reservoir straddles the divide between the newly formed igneous and metamorphic rocks of the Rocky Mountains and the sedimentary rocks laid down when the area was an inland sea.
This geology meant there was not enough clay in the valley to form the water-tight seal in the dam, unlike other reservoir sites holding Horsetooth Reservoir and Carter Lake. Instead, project managers hired a panel of experts to explore other alternatives.
They returned with the suggestion to build an asphalt-core dam, similar to dozens found in Europe but not found in the US. An asphalt-core dam uses a metre-wide paved section of hydraulic asphalt at the centre of the dam to ensure water from upstream does not migrate downstream and ruin the dam.
Project managers then required the contractor for the project, Barnard Construction Co. Inc., to choose from a group of European contractors with experience in asphalt core production. The Swiss-based company WALO was brought on to produce the asphalt core and install it simultaneously to the rock-fill construction.
The main dam includes 9.2Mm3 of material quarried from the igneous and metamorphic rock on-site and crushed for its construction. At peak construction, 500 workers were at the site, with construction occurring 20 hours per day, six days each week. Maintenance periods occurred for four hours each day. Near the completion of construction, project managers discovered some of the quarried materials in the dam included highly soluble, low-grade uranium minerals. While such materials and quantities are treatable, they were a surprising find in the rockfill structure.
The main dam emerged as being taller than any of the Colorado-Big Thompson Project dams, at 107m high. The addition of a 12m tall saddle dam at the southern end of the reservoir will allow the reservoir to store 110Mm3 of water for use by more than 825,000 residents on Colorado’s fast-growing Front Range. Located adjacent to existing C-BT Project infrastructure, the reservoir required little additional conveyance to ensure efficient delivery to the communities it serves.
In 2026, crews from the Municipal Subdistrict’s parent organisation, Northern Water, were able to place roughly 2.2Mm3 of water into Chimney Hollow Reservoir that had been collected in the winter of 2024-25 and stored in Lake Granby. That placement allowed the construction contract with Barnard Construction Co. Inc. to be declared complete. In future months, continued testing of the water in Chimney Hollow Reservoir will occur, and the 12 participants will decide the next course of action before any water is delivered to the communities receiving it. While snowpack and water resources will determine the date for opening the reservoir to public recreation use and large-scale deliveries, the reservoir is now in place to help Northeastern Colorado meet its needs well into the 21st century.
Report by Jeff Stahla, public information officer at Northern Water
