Chimney Hollow Reservoir Nears First Fill Operations

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Chimney Hollow Reservoir Nears First Fill Operations

CHIMNEY HOLLOW RESERVOIR – Beginning in August, crews will start to direct water through the valve house at the base of Chimney Hollow Dam and into the inlet-outlet structure at the bottom of the reservoir, starting a multi-year process to fill the reservoir.

Construction crews have been working for the past four years to build the largest new dam in the United States in 25 years. The project will bring much-needed water supply resilience to 12 participating entities serving hundreds of thousands of residents in Broomfield, Boulder, Larimer and Weld counties.

Construction has featured the second asphalt-core dam to be built in the United States, and by far the largest. Two-and-a-half miles to the south of the main dam, a 40-foot clay-core saddle dam has been built to increase the capacity of Chimney Hollow Reservoir by 27,000 acre-feet, or 30 percent.

As with any major public works project, construction teams and project managers had to confront challenges caused by weather, geologic conditions and more.

One challenge during construction arose through water quality sampling at Chimney Hollow Reservoir that revealed the presence of uranium-bearing minerals in water held at the site.
Mineralized uranium naturally occurs in some of Colorado’s geological formations, and uranium exposure in small amounts is common for Colorado residents. Naturally occurring mineral uranium is not a significant source of radioactivity. Uranium in drinking water is monitored by state and federal regulators for its health effects through long-term ingestion. It is occasionally found in groundwater, and some drinking water systems treat for it in their raw water supply.

The uranium at Chimney Hollow is naturally occurring in the granitic rock formations that were quarried to supply material for the dam. As the reservoir fills, these rocks will be in direct contact with water, leading to the leaching of uranium into the water. This issue will initially require close monitoring and management when filling Chimney Hollow Reservoir. The leaching of uranium into water is expected to decrease over time, although it is currently uncertain how long this process will take. No water will be released from Chimney Hollow Reservoir until further assessments are complete.

Dam safety and other tests will continue over the next two to three years as mitigation strategies are being pursued and implemented.
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For further information, contact Jeff Stahla, Public Information Officer, at [email protected].
About Northern Water

Northern Water, a public agency created in 1937, provides water for food production and municipal, domestic and industrial uses for more than 1 million people in Northeastern Colorado via the Colorado-Big Thompson Project, Pleasant Valley Pipeline and Southern Water Supply Project. Northern Water also generates hydropower at two sites and provides water quality services throughout the region. Its Municipal Subdistrict delivers water through the Windy Gap Project. Northern Water and the Municipal Subdistrict are currently planning the Northern Integrated Supply Project to enhance Front Range water supplies for the future, as well as constructing Chimney Hollow Reservoir which is the key component of the Windy Gap Firming Project. Learn more at?www.northernwater.org.