Project header_Chimney Hollow

Once completed, the reservoir will be impounded by the largest asphalt core dam in the United States.

Funded by 12 northeastern Colorado water providers, Chimney Hollow Reservoir will add resilience to the water supply for more than 500,000 residents. To build the reservoir, Barnard is constructing a zoned rockfill main dam, zoned rockfill saddle dam, reinforced concrete inlet/outlet structure, concrete-lined tunnel, concrete spillway, and water distribution facilities. Typically, a rockfill dam has a clay core, but due to a lack of available onsite clay, the main dam was designed with an asphalt core. Dams with asphalt cores are common in Europe and Canada, but less common in the United States–the Chimney Hollow main dam will be the second of its kind in the country. To supply rockfill, the onsite quarry can produce up to 63,000 tons of aggregate per day, claiming the title of Colorado’s largest mining operation. When construction wraps up in 2025, the reservoir will provide critical water supply and recreational opportunities to the communities it serves.


Location: Larimer County, Colorado

Owner: Windy Gap Firming Project Water Activity Enterprise

Engineer: Stantec

Contract Type: Fixed Lump Sum (competitive bid); Fixed Unit Price (competitive bid)

Status: Currently under construction


  • 12.5 million CY, 355-ft.-tall zoned rockfill main dam with hydraulic asphalt core
  • 40-ft.-tall zoned rockfill saddle dam with clay core
  • 3,600-ft.-long concrete spillway
  • 7,000 LF of 72-in.-dia. conveyance piping and valves
  • 2,000-ft.-long concrete-lined tunnel
  • 65-ft.-tall reinforced concrete inlet/outlet structure
  • 3.8 miles of permanent access roads
  • Valve house to regulate flows to/from different reservoirs
  • Temporary cofferdam and stream diversion
  • Foundation curtain grouting

90,000 acre-ft. water storage

4 million tons of aggregate processing

3.1 million CY of excavation

Project footer_Chimney Hollow