Sept. 9, 2025

System to get Water in and Out of Chimney Hollow Nearly Complete

The water conveyance system used for delivering water in and out of Chimney Hollow Reservoir is constructed and nearly complete. About 99 percent of the testing is completed and crews are finishing that work in September.  

The process of getting water to Chimney Hollow Reservoir begins on the West Slope. Just west of Granby sits Windy Gap Reservoir, where water is collected and pumped 6 miles via pipeline to Lake Granby. From there, existing Colorado-Big Thompson Project infrastructure conveys Windy Gap water through Shadow Mountain Reservoir and Grand Lake to the Adams Tunnel, which passes under Rocky Mountain National Park.   

After traveling 13.1 miles through the tunnel to the East Slope and then through four reservoirs and three power plants, water will make its way in and out of the Chimney Hollow Reservoir through the inlet/outlet works, which includes C-BT Project interconnections, the conduit, valve house, the inlet/outlet tunnel and the inlet/outlet tower located within the right (east) abutment. 

Crews are working on the finishing touches of the tunnel's downstream portal, which includes placing sculpted shotcrete and staining it to resemble the natural look of the rock.
  • C-BT Interconnect at Bald Mountain: Completed in April 2025. 
  • Chimney Hollow Conduit: Completed in September 2024.  
  • Valve House: Crews recently finished the epoxy flooring, as well as the coating of the pipes and valves. The control room inside the valve house is finished. On the outside, the only work that remains is minimal grading and placing road base down in the small parking lot that Northern Water staff will use during routine maintenance and operations of the reservoir. Eventually, this area will also be reseeded.  
  • Tunnel: Crews are working on the finishing touches of the tunnel's downstream portal, which includes placing sculpted shotcrete and staining it to resemble the natural look of the rock. Shotcrete took about a month to complete, and staining began at the end of August. Following completion of shotcrete, crews will finish the connections to the portal wall face, which includes a man door and a connection to the HVAC system to provide air circulation to the tunnel. 
  • Tower: Last month, crews finished up work on the tower, which included completing the testing and placing rip rap around the base.  

From Chimney Hollow Reservoir, water is released into Flatiron Reservoir where it can be delivered to the 12 participants using the existing C-BT distribution system that spans across Northern Colorado.