On March 15, crews at the Chimney Hollow Reservoir Project connected the downstream and upstream portions of the inlet/outlet tunnel, which will play a critical role of filling the reservoir and making deliveries to water users.

Although at a much smaller scale, it was a vivid reminder of another tunnelling milestone in Northern Colorado eight decades ago. On June 10, 1944, crews working from both the West and East slopes holed through the Alva B. Adams Tunnel, the 13.1-mile-long tunnel that enables Upper Colorado River Basin water to flow beneath Rocky Mountain National Park to East Slope water users. The Chimney Hollow tunnel goes through bedrock east of the main dam.

The Chimney Hollow milestone was just under two years in the making; crews performed the first cut on the initial portion of the tunnel on April 4, 2022. That alignment, called the downstream segment because it runs from the centerline of the dam to its entrance near the valve house, required about seven months of excavation. Crews then headed to the upstream side of the dam to work on the portion of the tunnel that will connect to the inlet/outlet structure in the reservoir.

Throughout the next year, crews will install a reinforced concrete liner, pipeline and mechanical equipment in the tunnel. The entire inlet/outlet tunnel will be completed by March 2025 and ready for water to start flowing into Chimney Hollow Reservoir in late summer of 2025.