Dec. 19, 2025

Chimney Hollow Reservoir Project Construction Complete

Marking a major milestone, 2025 was the year construction wrapped up on Chimney Hollow Reservoir—the tallest new dam built in the United States in the past 25 years. Construction milestones continued steadily throughout 2025. In February, crews completed the spillway and received the final two valves on site. By April, the last quarry blast occurred, and all leakage testing on pipes and valves was successfully completed. In July, final testing wrapped up, the crusher was demobilized, and the main dam received its final asphalt layer. By August, the main dam was complete. Then, in late October, the Colorado Division of Water Resources Dam Safety Branch performed its final inspection of the structure. On Dec. 19, just six years after Barnard Construction signed the contract to build the project, the Chimney Hollow Reservoir Project reached substantial completion, which marks a historic milestone for the project. Substantial completion means that the project is ready for its intended use and responsibility shifts from the contractor to the owner. 

Throughout the course of the four-year construction period, the project saw numerous milestones, including:  

  • 14.5 million cubic yards of material from the quarry (four times the volume of Hoover Dam)
  • Over two miles of steel pipe installed and welded
  • 80,000 cubic yards of concrete produced at batch plant
  • 512 lifts of asphalt laid to create the main dam’s impermeable barrier
  • At peak production, over 500 employees on site per day 

Barnard demonstrated an unwavering commitment to safety throughout four years of construction on Chimney Hollow. With over 3.4 million work hours logged, the project achieved an average Total Recordable Incident Rate (TRIR) of 1.4, significantly lower than the industry average of 2.6. Impressively, Barnard maintained a TRIR of less than 1.0 for the last two years, with no fatalities and only one lost-time injury during the entire project. These results reflect a culture where safety is not just a priority—it’s a core value. 

The work completed over the last four years stands as a testament to the perseverance and dedication of everyone involved, not only during construction, but also throughout the two decades of permitting and planning that made this project a reality.  

Although initial plans called for water placement in the reservoir to begin in November, Northern Water and project participants are continuing water quality assessments near the site to address the presence of mineralized uranium identified earlier this year in the dam’s embankment. This work, which must be completed while the reservoir remains empty, will require additional time. The naturally occurring uranium was found in granitic rock quarried for construction of the Chimney Hollow Reservoir dam. Mineralized uranium is a common feature in certain Colorado geological formations. 

As the reservoir begins to fill, close monitoring and management will be essential, as the exposed rocks will come into direct contact with the water, leading to some uranium leaching. The leaching process is expected to decrease over time, although it is currently uncertain how long it will take. Staff and project participants are working diligently to collect data and explore operational strategies to address this temporary challenge. No water will be delivered from Chimney Hollow Reservoir until assessments are complete and a mitigation plan is developed to ensure a safe raw water supply.