March 9, 2023

Why is Chimney Hollow Reservoir Needed?

The Chimney Hollow Reservoir Project is part of a long-term water supply plan that is critical to support a number of Northern Colorado’s vibrant and growing communities. The project’s participants recognize the importance of water conservation and have already made considerable progress to reduce water consumption per person by more than 25 percent since 1988. They accomplished this through supply-side measures such as improving facility efficiency, as well as on the demand side by implementing tiered and metered billing, outdoor watering limitations and implementing reuse systems for gray water.  

Despite these efforts, conservation measures alone cannot meet future residential water demands. The project participants are expected to see a combined 2050 population exceeding 825,000 people – more than double their combined populations in 2005. Once built, Chimney Hollow Reservoir will provide dedicated storage to ‘firm’ Windy Gap water rights and ensure a reliable supplemental water supply for about 825,000 people in Northern Colorado. 

Spillway at Lake Granby.

The Windy Gap Project, which is operated by Northern Water’s Municipal Subdistrict, is located in Grand County on Colorado’s West Slope. The project consists of Windy Gap Reservoir (with a capacity of just 445-acre-feet) on the Colorado River, a pump plant and a six-mile pipeline to Lake Granby. Windy Gap water is pumped from the Colorado River and stored in Lake Granby, part of the Colorado-Big Thompson Project, before it is delivered to water users on the East Slope via C-BT infrastructure.  

The Windy Gap Project was designed to deliver an average of 48,000 acre-feet of water per year to Municipal Subdistrict participants. However, during wet cycles, Lake Granby is often full of C-BT Project water, leaving little or no space to store Windy Gap water.  As the C-BT Project’s largest reservoir, Lake Granby’s highest priority is to store C-BT water, with Windy Gap water storage being a lower priority.  

Currently, if there is no excess storage capacity in Lake Granby, the Windy Gap Project cannot operate or store any water. Additional reservoir storage specifically dedicated to Windy Gap Project water has been contemplated since the project’s inception. Today, nine municipalities, two water districts and a power provider are collaborating with Northern Water’s Municipal Subdistrict to build a new reservoir and make Windy Gap water deliveries more reliable.  

2023 Snowpack Projections  

January’s above-average snowfall was followed by a persistently drier February. In recent weeks, however, the pattern has become more active again. Currently, Northern Water’s SNOTEL stations (snow telemetry stations, or backcountry weather stations that measure snow) average at 105 percent of median on the West Slope and 115 percent of median on the East Slope. The Upper Colorado River Basin snowpack that feeds Lake Granby is at 115 percent of median.  

Current projections show a slightly less than 50 percent chance of water spilling at Lake Granby this year.