Firming Water Supplies from the Original Windy Gap Project

The Windy Gap Project was first proposed in 1967 by the cities of Boulder, Estes Park, Fort Collins, Greeley, Longmont and Loveland. Built between 1981 and 1985 and located on the West Slope near Granby, Windy Gap consists of a diversion dam on the Colorado River, a pump plant and a 6-mile pipeline to Lake Granby, the largest storage reservoir in the Colorado-Big Thompson Project system.

The Windy Gap Firming Project, of which Chimney Hollow Reservoir is a main component, will improve the reliability of, or firm, water supplies from the original Windy Gap Project, which started delivering water in 1985. It was understood the original Windy Gap Project would require additional storage to achieve firm water supplies. The Windy Gap Firming Project has been reviewed under the federal National Environmental Policy Act. This review started in 2003. NEPA and Colorado's requirement for a plan focused on fish and wildlife are among several processes in place to identify mitigation measures that will address the project's impacts. 

  • 2003
    Project participants enter the federal permitting process and U.S. Bureau of Reclamation holds three formal "scoping" meetings.
  • 2005
    U.S. Bureau of Reclamation published two reports, one describing the purpose and need, and one identifying a range of alternatives that could meet the needs.
  • 2008
    U.S. Bureau of Reclamation publishes the Draft Environmental Impact Statement.
  • 2009
    Municipal Subdistrict offers West Slope benefits to facilitate project implementation.
  • 2011
    State officials approve the fish and wildlife mitigation plan and voluntary enhancement plan
  • 2011
    U.S. Bureau of Reclamation publishes the Final Environmental Impact Statement.
  • 2012
    Grand County and the Municipal Subdistrict board approve agreements to create improvements to the Colorado River. Grand County issues 1041 permit.
  • 2014
    U.S. Bureau of Reclamation issues its Record of Decision and signs a carriage contract to transport water to Chimney Hollow Reservoir.
  • 2016
    Colorado issues a 401 Water Quality Certification.
  • 2017
    U.S. Army Corps of Engineers issues its final Record of Decision.
  • 2019
    December 2019
    The Board of Directors of the Northern Water Municipal Subdistrict chooses a contractor to build Chimney Hollow Dam. Barnard Construction Inc. of Bozeman, Mont., will enter into a $485.4 million contract that calls for the construction of a 355-foot-tall asphalt-core dam in the valley west of Carter Lake in southern Larimer County.
  • 2020
    Dec. 10, 2020
    Federal Court rules in favor of the Windy Gap Firming Project, clearing the way for construction of Chimney Hollow Reservoir near Berthoud.
  • 2021
    April 21, 2021
    The Subdistrict reaches an agreement with several environmental groups settling the lawsuit and appeal. The $15 million settlement will benefit aquatic habitat on the West Slope. The settlement will allow construction of Chimney Hollow Reservoir beginning in August 2021.
  • Aug. 6, 2021
    Eight Northern Water Directors shovel dirt at the Chimney Hollow Groundbreaking in August 2021.
    The Northern Water Municipal Subdistrict breaks ground on Chimney Hollow Reservoir, culminating a 20-year permitting process. Chimney Hollow Reservoir is a key component for these Windy Gap Firming participants: Broomfield, Platte River Power Authority, Loveland, Greeley, Longmont, Erie, Little Thompson Water District, Superior, Louisville, Fort Lupton, Lafayette and Central Weld County Water District. Each of the reservoir project participants that provide residential water service has committed to reduce per capita water supply through water conservation.
  • Aug. 16, 2021
    Aerial view of the Chimney Hollow Valley looking north toward Flatiron Reservoir.
    Northern Water’s Municipal Subdistrict issued a Notice to Proceed to Barnard Construction Co. Inc. to begin construction of Chimney Hollow Reservoir. Environmental and mitigation efforts have also begun in the Upper Colorado River basin.
  • Oct. 1, 2021
    First blast at the Chimney Hollow site on October 1, 2021.
    Crews conduct the first blast at the Chimney Hollow Reservoir construction site. Blasts occur regularly, often multiple times a week, for the duration of the project.
  • 2022
    January 2022
    Foundation grouting begins on the main dam at Chimney Hollow Reservoir.
  • Feb. 9, 2022
    Bridge over the Flatiron Penstocks that will provide recreation access when the reservoir opens to the public.
    In what is described as a “milestone” day, crews install a bridge to connect two sections of a new Larimer County access road at the Chimney Hollow Reservoir construction site.
  • Feb. 10, 2022
    Crews working on the plinth under plastic during the cold, winter months.
    Initial placement of the Chimney Hollow main dam plinth begins.
  • April 4, 2022
    Man spraying water while machine cuts into the downstream portion of the tunnel on April 4, 2022.
    Tunneling of the Chimney Hollow Reservoir inlet/outlet infrastructure gets underway.
  • July 7, 2022
    Four construction workers guide in a 72-inch butterfly valve that is being held by a crane.
    Crews place the first of 61 valves on-site as part of the conduit that will bring water into the reservoir. This valve is located in the valve vault on Bald Mountain approximately 700 feet above the top of the main dam and will provide isolation between the inflows to Chimney Hollow and the Colorado-Big Thompson Project. The 72-inch diameter valve weighs 23,000 pounds.
  • Aug. 1, 2022
    Man guiding in a piece of pipe for the Chimney Hollow Conduit.
    First piece of pipeline for the conduit installed.
  • Aug. 23, 2022
    Dignitaries from across the region gather to celebrate the start of construction at the Colorado River Connectivity Channel located in Grand County. Led by U.S. Sen. Michael Bennet, leaders of public agencies and private non-governmental organizations extol the value of the project that will reconnect two segments of the Colorado River above and below Windy Gap Reservoir.
  • Sept. 15, 2022
    Crews placing the wye in place at the Bald Mountain Interconnect that will allow water to be diverted into Chimney Hollow Reservoir.
    A shutdown of the Colorado-Big Thompson (C-BT) Project goes into effect as crews cut into the existing C-BT infrastructure at the Bald Mountain Tunnel. A 126-inch steel pipe with a 72-inch steel offtake (known as a wye) is tied-in so that future water deliveries can be made to Chimney Hollow Reservoir.
  • Oct. 15, 2022
    Crews place forms on the asphalt core of the main dam to hand-place a lift of the aslphat core.
    Crews place the first section of the asphalt core. The asphalt is placed in 10-inch lifts until it reaches the top of the dam (approximately 350 feet). Rockfill occurs concurrently to stabilize the asphalt core, covering the plinth.
  • November 2022
    Crews complete the main dam rock excavation after 15 months of work on this component.
  • 2023
    March 10, 2023
    Crews place first spillway slab.
  • March 31, 2023
    First valve house floor slab placed. The slam is 6.5 feet thick with about 280 cubic yards of reinforced concrete per slab.
  • March 31, 2023
    Barnard Construction surpasses 1 million hours at the Chimney Hollow Reservoir construction site.
  • April 1, 2023
    Crews begin full production on site with double shifts. All project components have begun.
  • April 11, 2023
    The main dam reaches original grade (elevation 5,545 feet), 50 feet up from the plinth.
  • April 21, 2023
    Chimney Hollow conduit reaches halfway point.
  • April 27, 2023
    One million cubic yards of zone 4 embankment placed on the main dam.
  • May 2, 2023
    Crews place 100th lift of asphalt on the main dam.
  • June 26, 2023
    The main dam at Chimney Hollow Reservoir hits 100 feet.
  • Oct. 25, 2023
    First water flowed through the Colorado River Connectivity Channel. The new channel around Windy Gap Reservoir hydrologically and ecologically reconnected two segments of the Colorado River for the first time in approximately 40 years.
  • November 2023
    Quarry blasting hit peak production supplying the approximate 62,000 tons of material daily. This material is keeping crews busy dumping a 100 ton load every two minutes, 20 hours a day, six days a week for the next two years.