Gross Reservoir Expansion: Denver's Dam Transformation and the Battle Over Water Security

Denver's Gross Reservoir expansion is the centerpiece of a plan to secure the city’s water future by enlarging the dam itself. The project envisions building a new 471 ft concrete dam atop the existing 340 ft structure using roller compacted concrete, adding about 77,000 acre-feet of capacity (from 42,000 to 119,000 acre-feet) and supporting 72,000 homes per year. Construction began in 2022, with 118 steps rising on the dry south side to form the new crest, spillway, and control buildings. But the project has faced a legal setback in 2025 when a federal judge paused construction after environmental groups argued the expansion would harm the Colorado River system. The plan continues to face regulatory hurdles, environmental mitigation efforts, and questions about cost and timing.

Introduction

The Gross Reservoir expansion in Colorado is a megaproject designed to secure Denver's water supply against growing demand and climate-driven risks. The B1M takes a close look at the engineering feat of upgrading the dam itself rather than building a separate reservoir, and at the climate and policy context that has made the project both necessary and controversial.

Project Overview

Originally completed in the 1950s, the Gross Dam created Gross Reservoir to serve the Denver metropolitan area. The plan approved in 2020 calls for expanding capacity from about 42,000 acre feet to around 119,000 acre feet, an increase of nearly three times. The expanded reservoir would be able to supply roughly 72,000 homes per year. The approach is to raise the dam by stacking a new concrete structure on top of the existing gravity dam, effectively converting a gravity dam into an arch dam to improve stability and water containment.

Engineering Approach

Conventional concrete construction built in large blocks is the historical method; in this project, the team uses roller compacted concrete RCC. RCC places concrete by dumping, spreading with bulldozers, and then roller-compacting the surface. RCC has lower curing temperatures and reduced embodied carbon compared with traditional concrete, and it reduces cracking risk because the material behaves as a single dam when placed against the old structure. This makes the Gross expansion the largest RCC dam raise in the world.

Construction Details

Construction began in 2022. By mid 2025 crews had reached the top of the original dam, which sits high in the mountains about 7,000 feet above sea level. The project increases the height by about 131 feet on top of a 340 foot tall dam, yielding a crest about 471 feet above the foundation. The new dam includes a crest, control building, spillway railing and a bridge across the spillway. A total of 118 steps are being built, each four feet tall and set back two feet from the one below. These steps form the south facing dry side needed for stability. The crew overlays more than 600,000 cubic yards of conventional concrete with more than 700,000 cubic yards of RCC, a scale not previously achieved in dam construction.

Materials and Environmental Considerations

Engineers emphasize the mix design, placement temperatures, and how RCC bonds to the original dam as critical to ensure that the finished structure acts as a single dam. The RCC approach allows lower curing temperatures, faster setting, and reduced carbon emissions, while potentially reducing cracking. However, the project triggers significant environmental concerns. About 200,000 trees must be removed, wetlands are being created and preserved to offset losses, and water quality must be protected. Denver Water argues that storing extra water in the expanded reservoir would help direct more water into adjacent creeks and improve fish habitats, representing a net environmental benefit with thoughtful mitigation.

Timeline, Funding and Operations

The expansion was approved in 2020, with a stated price around $531 million. The price may rise due to legal costs and additional permitting needs. Denver Water is financed by ratepayers and related revenue rather than tax dollars, and it already operates a hydro plant at Gross Dam since 2007. The finish line is a top-out in 2026, completion of spillway and supporting buildings in 2026, and readiness to fill the reservoir around 2027 or later, pending new permits. The 5 year filling period would require a string of environmental and regulatory approvals that remain unresolved after the April 2025 injunction and subsequent legal developments.

Controversy and public debate

Campaigners, including groups such as Waterkeeper Alliance, Sierra Club and Save the Colorado, argue the expansion would drain more water from the Colorado River and worsen drought conditions in the basin, which is already in crisis. They contend the Army Corps of Engineers did not adequately address alternatives and environmental impacts. Denver Water counters that the expansion is a mitigation measure and that the project includes habitat restoration, wetlands creation, and other measures to offset land clearing. The court decision in 2025 reflected this David versus Goliath dynamic, underscoring the contentious nature of large-scale water infrastructure in the American West.

Conclusion

Whether the Gross Reservoir expansion is ultimately a net win for Denver, Boulder County, and the broader Colorado River system remains debated. The engineering achievement is undeniable, as is the need for a resilient water future. The project has already pushed the boundaries of dam construction practice and demonstrates how critical infrastructure projects intersect with environmental policy and public sentiment in the 21st century.