Gross Reservoir Expansion: Transforming a Gravity Dam into an Arch Dam in Colorado

The B1M investigates the Gross Reservoir expansion outside Denver, a bold plan to enlarge water supply by raising the dam and building an arch dam atop it. Engineers are using roller compacted concrete to overlay the existing gravity dam, creating the world’s largest RCC raise and boosting capacity significantly. The project faces fierce environmental opposition, court injunctions, and a complicated permit and filling timeline. It blends impressive construction with contentious environmental concerns as Colorado weighs its water future.

Overview

Outside Denver, Colorado, the Gross Reservoir project represents a major hydraulic and civil engineering endeavor. The original Gross Dam, built in the 1950s to serve the Denver metropolitan area, held about 42,000 acre-feet of water. To meet growing demand and improve resilience to drought and natural disasters, Denver Water sought to expand the reservoir by raising the dam and adding a new structure on top. The plan calls for a 131 ft increase in height atop a 340 ft tall dam, resulting in a final dam height of about 471 ft. The expanded capacity would reach roughly 119,000 acre feet, or about 150 million cubic meters, enough to serve tens of thousands of homes and improve regional reliability. Construction began in 2022, and by mid-2025 crews had reached the height of the original dam, located some 7000 ft above sea level.

Engineering Challenge

The expansion is notable for transforming a gravity dam into an arch dam, a process not previously undertaken at this scale. Instead of building with conventional concrete blocks, Denver Water chose roller compacted concrete RCC. Concrete is dumped from trucks onto the site, spread with bulldozers, and then compacted by rollers to form a continuous, large-scale mass. The project overlays more than 600,000 yards of conventional concrete with over 700,000 yards of RCC, making it the largest RCC dam raise in the world. The RCC mix requires strict temperature control, compatibility with the old dam, and precise design to ensure the new and old sections act as a single structure. To achieve the arch dam concept while stabilizing the south-facing side, 118 concrete steps rise from the base to the crest, each 4 ft high and offset from the one below. A crest, control buildings, spillway improvements, and a new bridge are needed as part of the upgrade. The result aims to produce a safer, more efficient water barrier capable of withstanding extreme weather and climate variability.

Environment and Community Context

Denver Water presents mitigation measures to reduce environmental impact, including wetland creation and habitat improvements to offset losses from forest clearing and land inundation. The project requires removing around 200,000 trees and managing water quality during construction. Campaigners and environmental groups argue that enlarging a major water project in the upper Colorado River system could negatively affect downstream flows and regional ecology, given ongoing drought and climate change concerns.

Legal and Funding Landscape

In 2025 a US District Court injunction paused construction after environmental groups challenged the project on environmental impact analyses and permitting processes. The Army Corps of Engineers issued permits, but critics argued alternatives and impacts were not adequately addressed. The judge briefly halted the project, then reversed the decision after concerns about flooding if the dam were left unfinished. Despite the legal back-and-forth, officials anticipated continuing work pending new permits and a decision on reservoir filling, a process expected to take around five years once approved. The project has been funded by Denver Water through ratepayer contributions and related revenue streams, not direct tax dollars, with the initial price tag cited around 531 million USD, though legal costs and additional permitting could alter that figure. The completion timeline points to a finish around 2026 for the top-out and 2027 for reservoir filling, assuming all approvals are secured.

What’s Next

As Colorado weighs the long-term water strategy for Denver and Boulder County, the Gross Reservoir expansion stands as a case study in balancing infrastructure scale, climate resilience, environmental stewardship, and community impact. The outcome will influence not only regional water security but also future engineering approaches to large-scale dam raises and the use of RCC in major infrastructure projects.