Feb. 4, 2025

Chimney Hollow Reservoir Enters Final Year of Construction

With the final full year of scheduled construction at Chimney Hollow in the books, many project components are finished or nearing completion. In 2024, crews safely worked almost 1 million hours with safety incident rates of about one-third of the national average. 

Here is a highlight of the activities for the year:

  • Dams: Significant progress on the main dam and saddle dam. The main dam reached just over 300 feet tall, which leaves only about 50 feet to go to the final height of 350 feet. Crews have placed about 5.7 million cubic yards of rockfill and 30,000 cubic yards of asphalt. The embankment of the 40-foot-tall saddle dam began in May and was completed in mid-November. About 45,000 cubic yards of clay was sourced on-site and placed on the dam, along with nearly 110,000 cubic yards of rockfill.
  • Grouting: The grouting program, which began in May 2022, wrapped up in fall 2024. The grouting team drilled 32 miles of holes injected with cement grout.
  • Inlet/Outlet Works: The Chimney Hollow Conduit and inlet/outlet tunnel were completed in 2024. Moving into 2025, crews will complete the valve house and inlet/outlet tower, as well as finish installation of conduit from the valve house through the tunnel.
Last lift of clay is placed on the Chimney Hollow Saddle Dam in November 2024
The final lift of clay is added to the saddle dam at the south end of Chimney Hollow Reservoir. The saddle dam was completed in late 2024.
  • Final grades: As work areas are completed, crews are constructing the final grade for the project, including removing temporary construction roads. Disturbed areas are covered with topsoil and revegetated with native seed mix for a successful establishment in future years.
  • Control systems: Northern Water staff are installing control systems for the new reservoir and preparing for operations once it is commissioned.

Despite some setbacks in 2024, including six adverse weather delays and four days lost to mandatory evacuations from nearby wildfires, the project is still a couple of weeks ahead of schedule. As construction wraps up in summer 2025, project managers plan to begin filling the reservoir, which is expected to take about three years. Larimer County will manage the reservoir’s recreation and is expected to begin construction of trails and parking lots after the reservoir is complete.