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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
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.
www.waterpowermagazine.com | September 2026 | 17 of water into Chimney Hollow Reservoir that had
The Colorado foothills west of Fort Collins with a water supply canal, a part of Big Thompson Project
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