Q&A on the Gross Reservoir Expansion Project
Completion of the dam-raising phase of the Gross Reservoir Expansion Project in Boulder County marks a significant engineering feat for Denver Water, one that highlights the utility’s ongoing work to ensure water security for 1.5 million people in Denver and surrounding suburbs.
As Denver Water marks the construction milestone, Jeff Martin, the program manager for the Gross Reservoir Expansion Project, gave an update on the work and discussed the benefits of the project designed to nearly triple the storage capacity of Gross Reservoir.
What milestone for the Gross Reservoir Expansion Project occurred in June?
Raising Gross Dam is an incredible engineering and construction feat that Denver Water has been pursuing for over 20 years. On June 3, 2026, Denver Water marked a major milestone by topping out the roller-compacted concrete portion of the expansion project, raising the dam 470 vertical feet from the valley floor to the top, 130 feet higher than the old dam.
What work is still left to do?
Denver Water has about a year and a half worth of work to complete the dam-construction phase of the expansion project. The remaining work includes adding 1 foot of protective, conventional concrete to the top of the roller-compacted concrete, installing a water gate, completing the spillway crest, building a bridge over the spillway itself, and finishing the stilling basin at the bottom.
What is the benefit of a larger reservoir for Denver Water?
The new reservoir is designed to boost Denver Water’s overall water storage capacity by 11%. The expanded reservoir is capable of holding an additional 25 billion gallons of water.
This project provides water security and reliability for 1.5 million people around Denver and our surrounding communities by nearly tripling the size of Gross Reservoir behind the new dam.
The northern part of Denver Water’s collection system needs more water storage because of an imbalance in its water storage system. Ninety percent of the utility’s water is stored on the south side of the collection system, while just 10% is stored on the north end. This project will help balance that by allowing Denver Water to store more water on the north side, which will help provide a secure water future to Denver.
How would a larger reservoir help during droughts?
Denver Water is coming off one of the worst snowpacks on record and one of the most memorable droughts in over 20 years. This project is a critical part of how Denver Water will manage through future droughts. The expanded reservoir will allow more water to be stored in our system, when it’s available, for use when it’s needed. This new storage will provide flexibility and resilience for how we collect, treat and deliver water to customers.
How does the project help Denver Water prepare for the impacts surrounding climate change?
As Denver Water plans for the uncertainty of climate change, including more variability and extremes in weather patterns, a bigger bucket allows us to capture and store more water, which provides more flexibility. In really wet years, Denver Water can capture more water to offset dry years. The bigger reservoir allows us the flexibility to maneuver and work with our changing climate.
How has Denver Water addressed the environmental impacts of the expansion project?
Denver Water worked with stakeholders on the West and East slopes of the Continental Divide to identify ways to make this project a net environmental benefit.
Over the 20-year process of permitting and building Gross Dam, we’ve made major commitments to provide environmental enhancements and mitigation for this project, including committing $30 million to over 60 different environmental enhancement projects to offset impacts of the Gross Reservoir Expansion Project.
One of the goals of expanding Gross Reservoir was to improve stream health on some of the streams the project touches, including stream restorations on the Williams Fork and Fraser rivers in Grand County and South Boulder Creek in Boulder County.
Denver Water also set aside ecologically sensitive fens and wetlands to compensate for the wetlands that were disturbed during the project.
Are there still legal issues surrounding the project?
As of July 2026, the Gross Reservoir Expansion Project is still the subject of ongoing litigation in the 10th Circuit Court of Appeals. Denver Water is confident the project will move through those challenges, and we'll see an expanded reservoir in the future. The timing for when we'll fill the expanded reservoir to its new, larger capacity will depend on the resolution of current litigation in federal courts, as well as water availability once the project is complete.