Gross Dam in Boulder County, Colorado, is undergoing a $565 million expansion, with the dam's height being raised to nearly triple the Gross Reservoir's capacity from 42,000 acre-ft. to 119,000 acre-ft.

The Gross Reservoir serves the Front Range region of the Denver Metropolitan area along the Interstate 25 corridor, which includes the cities of Denver, Boulder and Fort Collins. The entire Denver Water system supplies 1.5 million people.

Denver Water photo

The dam's expansion is key to Denver Water's long-term supply plan.

Historically, there has been an imbalance in the north-south water reservoir systems, with 90 percent of the storage in the south system and only 10 percent in the north system where the Gross Reservoir is located. This disparity leaves the water supply vulnerable — if there is a failure in the south system, the north system doesn't have a sufficient water storage capacity to supply the entire region.

The expansion is designed to mitigate that risk and foster a greater resilience in the face of climate-related events or other catastrophes.

In addition, the expansion will enable the system to capture water during wet seasons and reserve it for future use. Before this improvement, excess water runoff flowed out of the area. As a dry, drought-prone region, Colorado was surrendering a precious resource.

Originally built in the 1950s, the Gross Dam was always intended to be a three-phase project, with raises implemented over time by the system's needs.

This current phase involves building 118 concrete steps, which raises the dam by 131 ft. The expansion brings the dam's full height to 471 ft., the width to 2,000 ft. and makes the Gross Dam the highest in Colorado. This is the largest dam raise project in the United States to date., according to Denver Water.

Kiewit Corp. and Barnard Construction Co. formed a joint venture to complete the project. The construction process is complex and involved an enormous amount of material and work, highlighted by these numbers:

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• 2 million tons of aggregate processing

• 230,000 sq. ft. of hydro-demolition

• 1 million cu. yds. of site excavation

• 90,000 cu. yds. of conventional concrete

• 23,000 sq. ft. of wire saw demolition

The raised dam will be steeper than the original version. Its base will span 300 ft., and the top narrows to just 25 ft. Each of the 118 steps is 4-ft. tall and 2-ft. wide.

The steps are comprised of roller-compacted concrete, which is being placed over the existing structure. This is considered an efficient method of installing concrete to accelerate the building process. The stepped design was essential during construction to support the trucks that held, transported and deposited the materials.

Bob Mahoney, Denver Water's chief of engineering, discussed the project's long-term impact.

"Four years of design, 20 years of permitting, three years of construction got us to the top of the existing dam," he said. "This project is going to give more security and resiliency to Denver Water's supply to serve our customers over many years to come."

Casey Dick, deputy program manager of the Gross Reservoir Expansion Project, described the process.

"The way we are raising the height of the dam is by using roller-compacted concrete," he said. "A critical part of the new construction is building the formwork. That's the blue framing that is used to hold the concrete in place until it cures … The original structure was built as a curved gravity dam.

Denver Water photo

"Now, we are taking advantage of that curved geometry to create what's called a thick arched dam in the center of the canyon. This arched style means the force of the water in the reservoir pushes up against the dam and into the canyon wall to give it its strength."

The crews have set up a batch plant on site to make the concrete being used in the dam. They are employing a large conveyor system to transport the concrete from the batch plant to the dam. The engineers designed the delivery system to accommodate the incline as the dam rises gradually higher.

In addition to the 118-step rise, other key features are being added. Two walls up the middle of the dam form a spillway channel, and thrust blocks were erected on the sides of the dam to provide additional support by extending the canyon walls upward to bolster the arched dam in the center.

Environmental concerns about the project were raised, including the loss of trees, depletion of the Upper Colorado River, urban sprawl and quarrying, which resulted in litigation. A judge issued an order to stop work in April of 2025 on the grounds of improper permitting, but the ruling was vacated, and construction resumed the following month.

To address ecological issues, the project includes a 5,000-acre-ft. environmental pool to increase stream flow and enhance the habitat for the flora and fauna in a 17-mi. long section of South Boulder Creek. It will be used to release water during low-flow periods to support fish populations.

Denver Water photo

"I understand the water storage requirements of the state need to be balanced with the environmental impacts the projects may have," state Rep. Karen McCormick said. "I know that the project has experienced many hurdles, and I was glad to hear that Denver Water was able to come to an agreed upon resolution with the local residents and communities ...

"I had some hesitancy about the project early on, but learning about the alternatives and how this was the best plan to ensure water availability for Coloradans into the future and how Denver Water has worked through each hurdle, I now am fully supportive of the project. The increased water storage and recreational opportunities will benefit us all."

Crews worked around the clock this fall to advance the project before winter weather made outdoor construction impossible.

Anticipated completion of the dam is expected in 2027. CEG