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What is the Gross Reservoir Expansion Project?

What is the Gross Reservoir Expansion Project? Environmental Benefits

The existing facility was designed as a phased project. The first phase, completed in the 1950s, is the current dam and reservoir, which supplies water to the north end of Denver Water’s system. The original plans called for two additional raises, with the timing determined based on needs in the system.

Raising the dam

In 2002, after a historic drought and the Hayman Fire, Denver Water identified the need to explore additional storage in its North System, which makes up only 10% of the utility’s total storage capacity, putting the entire system at risk.

After an extensive review process that initially included over 300 different options, the U.S. Army Corps of Engineers determined that expanding Gross Reservoir was the “Least Environmentally Damaging Practicable Alternative” to address Denver Water’s objectives of reliability, flexibility and supply. 

The Gross Reservoir Expansion Project, one part of Denver Water’s “all of the above approach,” will provide greater water security to the Front Range; reduce vulnerability in the face of climate change-driven disasters, such as droughts, floods and fire; and ensure flexibility by addressing the imbalance across Denver Water’s North and South systems, allowing the utility to provide water from multiple sources when needed. 

The project will raise the height of the existing dam by 131 feet. This will more than triple Gross Reservoir’s capacity, from approximately 42,000 acre-feet to 119,000 acre-feet.

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Of the additional 77,000 acre-feet added through the expansion, 72,000 acre-feet will be for Denver Water customer use and 5,000 acre-feet will go to an environmental pool, which will provide enhanced stream flow to a 17-mile stretch of South Boulder Creek, as well as additional storage for Boulder and Lafayette. 

This project will capture and store water in average and wet years. The additional water supply for Gross Reservoir will come from winter and spring runoff during the high runoff months of May, June and July. Water will not be diverted for this project during low-flow months and when water is scarce.  

Most runoff available to Denver Water during the wet winter of 2014 and spring 2015 flowed out of state because existing Denver Water reservoirs were full and there was no place to capture and store the excess water on the Front Range. If the reservoir expansion had been complete, the reservoir would have captured and stored 72,000 acre-feet of water by summer 2015.