Frequently Asked Questions about Chimney Hollow Reservoir

Where will Chimney Hollow Reservoir be located?

Chimney Hollow Reservoir, which is the main component of the Windy Gap Firming Project, will be located just west of Carter Lake and south of Flatiron Reservoir, southwest of Loveland, Colo. in Larimer County.

How large is the reservoir?

Chimney Hollow Reservoir will store up to 90,000 acre-feet and will have 740-acre surface acres at capacity. Water will be stored behind a 350-foot-tall dam. Chimney Hollow Dam will be the third tallest dam in the State of Colorado and one of the tallest on the Colorado’s Front Range.

Will Chimney Hollow Reservoir be open to the public?

Yes, Larimer County will manage the Chimney Hollow Open Space Public Recreation Area. The open space will include over 2,000 acres of forest in addition to the 740-acre reservoir. The public open space will provide a day-use recreation area with hiking, horseback riding, fishing and non-motorized boating.

How much is this project going to cost and who is paying for the project?

The project is estimated to cost $670 million. The project will be fully paid for by the 12 project participants.

Is this project needed?

Yes! This project is part of a long-term water supply plan that is critical to support our growing and vibrant communities. Chimney Hollow Reservoir will provide dedicated storage to ‘firm’ Windy Gap water rights and ensure reliable supplemental water supply for about 825,000 people in Northern Colorado.

Why has this project taken so long?

Project participants entered the formal federal permitting process in 2003. The federal environmental permitting process consists of a thorough analysis of project needs, impacts and ways to mitigate those impacts and that takes time.

Has the project met all the permitting requirements?

All state and federal permits have been issued and construction began in August 2021.

How will this project benefit the Colorado River?

The Windy Gap Firming Project, in which Chimney Hollow Reservoir is a main component, includes mitigation of potential impacts from the project and voluntary enhancements to improve the health of the Colorado River. Among other things, these programs include the following measures:

  • Water quality and nutrient mitigation for the Colorado River
  • Upper Colorado River Endangered Fish Recovery Program
  • Water quality and temperature monitoring in the Colorado River
  • Colorado River Connectivity Channel
  • Colorado River Habitat Project
  • Learning by Doing program to benefit the Colorado River

The project also provides water and water storage for Grand County to use for environmental purposes. Grand County values the water and storage at approximately $45 million, which is a very substantial contribution by the project to the long-term environmental health of the Colorado River.

The project includes operations to improve the health of the Colorado River. Pumping operations will be curtailed to maintain temperature standards for fish health, and ‘flushing flows’ will be provided to move sediment and improve the aquatic habitat of the Colorado River.

Why do we need another dam? Can’t we just conserve water instead?

Project participants recognize the importance of water conservation. The participants have diversified, substantive conservation programs that they continue to evaluate and strengthen. The project participants have already made considerable progress by reducing water consumption per person by more than 25 percent since 1988. They accomplished this through supply-side measures such as improving facility efficiency, as well as on the demand side, by implementing metered billing, outdoor watering limitations and implementing re-use systems for gray water.

Despite these efforts, conservation measures alone cannot meet future residential water demands. The project participants are expected to see a combined 2050 population exceeding 825,000 people – more than double what they had in 2005.

How long will construction last?

We anticipate that construction will take about four years. Based on our current construction schedule, we will begin filling Chimney Hollow Reservoir in 2025.

How long will it take to fill the reservoir once construction is complete?

We expect it will take about three years to fill the reservoir. 

What is an asphalt core dam, and is it safe? Why aren’t we using a ‘normal’ design like Horsetooth Reservoir?

Asphalt core dam technology was originally developed in the 1960s in Germany, Austria and Norway for areas that lacked sufficient clays to construct clay-core dams.Asphalt core dams use a special hydraulic asphalt mix, unlike brittle paving asphalt, to create a flexible, very low permeability core. This technology is proven effective, with more than 200 asphalt core dams constructed worldwide having decades of safe operations. 

At the start of design for the Chimney Hollow Reservoir, an extensive clay-borrow geotechnical engineering study was undertaken that indicated that we did not have enough clay material in the valley to construct a clay-core dam. At that time, we hosted a workshop with our design engineer, the State of Colorado Division of Dam Safety and four independent technical consultants who unanimously agreed that the asphalt-core, rockfill embankment dam was the best and safest solution for the Chimney Hollow main dam. 

There is sufficient clay material in the valley to construct the saddle dam at the south end of the reservoir. The saddle dam will be a 40-foot tall, clay-core, rockfill embankment dam. 

Is this a zoned earthfill dam with internal drainage? 

The main dam is a zoned asphalt core rockfill embankment dam. No clay. The saddle dam is a clay core zoned rockfill embankment dam. Each of the respective dams are designed with appropriate internal drainage based on the embankment design.

Why was a Swiss contractor selected for the asphalt core instead of a U.S.-based company?

Our approach to procurement involved hiring a U.S. based General Contractor. However, we did require all prequalified contractors bidding on the job to prove to us that they understood the technical aspects of constructing a hydraulic asphalt core dam. Our general contractor, Barnard Construction, chose to team with WALO to gain that specialty knowledge and bring their experience to Chimney Hollow.

Will Chimney Hollow Reservoir have hydropower turbines?

There are no hydropower turbines within Chimney Hollow reservoir due to the nature of planned reservoir operation and project water rights. The reservoir is designed to act as a "bank account" for Windy Gap water. It will remain full a majority of the time with only a maximum of 30,000 acre-feet passing through on any given year. As a result, water will not move through the Chimney Hollow system at a volume that makes hydropower feasible. However, because Chimney Hollow utilizes existing Colorado-Big Thompson infrastructure, flows do pass through 3 hydropower plants while being delivered into Chimney Hollow. As a result, the net hydropower from delivering the water (after pumping etc.) is enough to power about 2,000 homes for a year.