Draining Antero Reservoir: Where does all that water go?

And 9 more facts about rehabbing Denver Water’s 100-year-old Antero Dam

On June 1, 2015, Denver Water had to empty Antero Reservoir to clear the way for significant repairs to the 100-year-old dam. Draining Antero is a major undertaking; the reservoir holds about 20,000 acre-feet of water, enough to supply approximately 50,000 households for one year in the Denver metro area. And such a project is bound to raise questions about the dam, the water supply and the impact on recreational fishing. Here are the answers to questions we thought Coloradans might ask:

  • 1. So what's wrong with the dam?

    Antero Dam is fully operational, but it’s old. The dam has been in service for 100 years, and this rehab project will help ensure that it can operate for another 100 years. That means bringing the dam in line with current standards of engineering and safety. It’s a big job, but a necessary one, at a cost of $20 million over several years.

  • 2. How long will the reservoir be empty?

    That depends. While the entire dam rehab project is tentatively scheduled to be completed in 2018, we expect to complete the phase that requires the reservoir to be drained by the end of 2015. Barring any weather or construction delays, refilling could begin as soon as spring of 2016. Generally, it takes from one to four years to refill the reservoir, depending on the amount of snowfall and timing of snowmelt. While it may be sooner, the safe bet is that the reservoir will return to its normal operation by late 2017, when construction ends.

  • 3. When you drain the reservoir, where does all the water go?

    We are recapturing the water drained from Antero Reservoir in Marston, the reservoir in our system that feeds Marston Treatment Plant. 

  • 4. Will draining the reservoir cause flooding or other safety concerns?

    Denver Water has opted to draw down the reservoir in a planned and managed way before construction to minimize, if not completely eliminate, any flooding or safety concerns. It will take approximately two months to empty the reservoir. We believe this to be a much safer course of action than allowing water to remain in the reservoir and risking the need for an emergency unplanned release during construction.

  • 5. Have you drained the reservoir before?

    Antero Reservoir was drained in the late 1990s to complete some repairs to the outlet works of the dam. It also was drained during the 2002 drought. Due to its shallow depth, Antero Reservoir has the highest evaporation rate of any of Denver Water’s reservoirs. In times of low water supply, moving the water to other reservoirs in Denver Water’s system reduces evaporation losses and makes the water available to customers.

  • 6. What if we go into drought while the reservoir is empty?

    We’re not losing the water from the Antero Reservoir. It will be stored in another reservoir, where it can still serve as a reserve water supply.

  • 7. If we have enough water in our system to drain a reservoir, why do you stress the need to conserve every year?

    We are storing the water drained from Antero Reservoir in our collection system. Antero Reservoir is a reserve water supply that Denver Water maintains for use when our water supplies run low. Regardless of conditions, it is important that we all use water efficiently.

  • 8. Will we still be able to use the park during construction?

    We will began draining the reservoir in June 2015. The park closed to the public on June 1, 2015, and will remain closed for the duration of the project. The park will re-open for recreation once the reservoir has been refilled and recreational opportunities, like fishing, bird-watching and camping, have been restored.  

  • 9. Will fishing be better after the project?

    While there are no guarantees, we expect the rehabilitation project to provide a long-term benefit to the fishery by allowing us to return the reservoir to a depth of 18 feet (except during drought periods). The reservoir has been operating at a reduced capacity since May 2011, when we lowered the reservoir by 2 feet to investigate the condition of the dam.

  • 10. What effect will this have on fishing at Antero and in other reservoirs in Park County?

    Park County has many other accessible and productive fishing locations for anglers, water enthusiasts and other outdoor lovers to enjoy. We encourage everyone to take advantage of those areas while Antero Reservoir is unavailable. Additionally, fish from Antero Reservoir will be relocated within the county, which may even improve fishing at some of these other locations.

    Check out this interactive map to see dozens of other recreational opportunities in Park County.

    Questions regarding fishing at Antero can be directed to Colorado Parks and Wildlife at 303-291-7227.

Additional project background:

  • What will the construction look like for the Antero Dam Rehabilitation Project?

    The project will be done in phases. Phase 1 wrapped up in June 2014. During this phase, Denver Water built a sand trench to filter the normal seepage from the dam.

    Phase 2, which began in the summer of 2014, involves grading the embankment and ensuring appropriate drainage. This phase is scheduled to be completed in the spring of 2015.

    Phase 3 will result in the installation of a barrier wall inside the dam. Construction is scheduled to start in August 2015 with a tentative completion date of June 2016.

    A fourth phase will involve the construction of a new spillway and improvements to the outlet works, and is tentatively scheduled to be completed in late 2017.

  • What is the timeline for the Antero Dam Rehabilitation Project?

    Completion of the project, refill and a return to normal reservoir operations will be heavily dependent on construction timelines and weather. The tentative timeline for completion of the project is:

    • August 2013: Project and phase 1 (filter trench) begins.
    • June 2014: Phase 1 ends.
    • May 2014: Phase 2 (embankment grading) begins.
    • June 2015: Phase 2 ends.
    • August 2015: Phase 3 (installation of the barrier wall) begins.
    • June 2016: Phase 3 ends.
    • Summer 2016: Phase 4 (new spillway and improvements to outlet works) begins.
    • 2017: Phase 4 ends.