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The dam was finished before the water-quality question was. Chimney Hollow Reservoir reached substantial completion on December 19, 2025, according to Northern Water’s Municipal Subdistrict financial report. The planned November introduction of water had already been postponed while the project’s participants investigated naturally occurring uranium in rock used to build the embankment.
This was a major addition to northern Colorado’s water infrastructure: a reservoir designed to hold 90,000 acre-feet, with a main dam rising 350 feet. Its purpose was to improve the reliability of Windy Gap water deliveries by adding storage. Physical capacity, however, could not become a dependable supply simply because construction crews had completed their work.
The construction milestones were substantial. In an account published September 15, Northern Water said crews had celebrated topping off the main dam on August 8. The work had taken more than 1,000 days and 3.3 million labor hours. Equipment that had supported construction, including the asphalt plant and aggregate-processing facility, was being removed as those activities ended.
The Municipal Subdistrict’s later report recorded completion of the spillway in February, the last quarry blast in April and a final inspection by Colorado’s dam-safety officials in late October. December’s substantial-completion milestone closed that construction sequence. It did not resolve the separate decision about how to fill and operate the reservoir.
Northern Water’s subsequent account of the investigation traced elevated uranium measurements back to unusually wet conditions in 2023 and another discharge event in 2024. Investigators suspected processed granitic rock in early 2025. Tests in February and March then demonstrated that uranium could leach from rock in the embankment into water.
The material was not uranium ore being mined for fuel. Northern Water described trace concentrations dispersed through cracks and fractures in the granite. The relevant issue was contact: a large volume of rock would be exposed to reservoir water. Small concentrations distributed across a large surface could still affect the chemistry of the stored water.
The district also disclosed a gap in the earlier screening. Its design-stage geochemical testing covered a panel of twenty metals, but uranium was not included. That fact did not establish, on its own, what any different testing program would have predicted. It did explain why the subsequent investigation required additional characterization rather than merely checking an original design value.
For project participants, the delay changed the transition from construction to operations. They needed data on how leaching would behave and a management plan before deliveries. Northern Water said the process was expected to diminish over time, but the duration remained uncertain. That expectation was an assessment to test, not a schedule on which users could rely.
The distinction remained visible in the following year. Northern Water reported introducing about 1,500 acre-feet in April 2026 for monitoring and commissioning, less than two percent of capacity. Its published water-quality update said water would not be released until a management plan was identified.
Chimney Hollow’s 2025 completion therefore represented a real engineering achievement with an unresolved operating condition. The reservoir had acquired its structure, pipes and valves. Converting those assets into reliable water storage still depended on understanding the rock that surrounded the water.
Sources
- FY2025 Municipal Subdistrict Annual Comprehensive Financial Report — Northern Water Municipal Subdistrict.
- Chimney Hollow Main Dam Topped Off — Northern Water · 2025-09-15. Original archived release; its old link was unavailable when this archive was published.
- Chimney Hollow Water Quality — Northern Water.
- Reservoir on Track to Finish in Late Summer 2025 — Northern Water · 2024-02-12. Original archived release; its old link was unavailable when this archive was published.
- 2025 Annual Report — Northern Water.
- Chimney Hollow project timeline — Northern Water.