On July 18, 2025, a nearly 100-meter-high wall will become operational in Colorado, at the bottom of a valley that conceals the Chimney Hollow reservoir, a $690 million project created to store billions of gallons of water and combat decades of drought in the American West.

On July 18, 2025, after construction began in 2021, the United States completed the Chimney Hollow Dam in northern Colorado, a A wall nearly 100 meters high at the bottom of a valley. Designed to submerge an entire area and form a new freshwater reservoir, the project marks the country’s return to large-scale water infrastructure projects after nearly four decades without new megadams, a period marked by strong environmental resistance and movements opposed to this type of infrastructure.

Over the past two decades, the American West has faced severe droughts, dwindling reservoirs, and increasing pressure on the Colorado River, a source of water for tens of millions of people in states such as Nevada, Arizona, Utah, California, and Colorado itself. In this context, Chimney Hollow was conceived as a centerpiece of a water security plan for more than 500 residents of 12 communities in the Front Range region, relying on a reservoir that will take approximately three years to fully fill. depending on above-average snow and rainfall.

Why did Colorado decide to build a nearly 100-meter wall at the bottom of a valley?

A nearly 100-meter wall at the bottom of a valley creates the Chimney Hollow Dam, a water reservoir for water security to combat drought in Colorado.

For much of the last century, the United States built more than 90 dams.

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From the 1980s onwards, however, a strong opposition movement halted virtually all new large projects, associating dams with the destruction of rivers, valleys and entire habitats.

Change begins with the combination of Population explosion in Colorado’s Front Range and prolonged droughts in the west.

While desert cities have begun to cope with rationing and reservoirs like Lake Mead and Lake Powell have fallen to historically low levels, municipalities on the east side of the Rocky Mountains project to double their population by 2050.

Meanwhile, much of the water remains concentrated on the western side of the mountain range.

Each spring, the thaw spills Billions of cubic meters of water heading towards the Pacific.while the more populated east tends to be drier.

The answer was to reposition this water on the map: by purchasing, in 2004, approximately 3.500 acres of former Hewlett-Packard land near Loveland, the Northern Water consortium and Larimer County initiated the plan that would culminate in… A wall nearly 100 meters high at the bottom of a valley. now occupied by the Chimney Hollow reservoir.

How does the Chimney Hollow dam and the hidden wall in the valley work?

A nearly 100-meter wall at the bottom of a valley creates the Chimney Hollow Dam, a water reservoir for water security to combat drought in Colorado.

With a total investment of 690 million dollarsOf which approximately $570 million was allocated to the main dam, Chimney Hollow does not follow the classic model of massive concrete.

This is a rockfill dam, in which compacted rocks form the main mass of the structure, while the center receives a special impermeable core.

The result is a rockfill wall with approximately 107 meters high, curved along the valley, capable of storing enough water to supply the Denver region for several years in scarcity scenarios.

This nearly 100-meter-high wall at the bottom of a valley transforms the former canyon floor into a retaining wall and the space above into a large artificial lake.invisible to those who only look at simplified maps, but crucial for the regional water balance.

To the south, an auxiliary dam of approximately 12 meters acts as an additional safety barrier, preventing the water level from exceeding limits considered safe.

Engineers describe the structure as an organism that combines rigid and flexible components, designed to absorb pressure, accommodate load variations, and withstand extreme events.

Hydraulic asphalt core and precision engineering

One of the challenges arose during the excavation phase: upon reaching the bottom of the valley, the teams discovered lack of clay in sufficient quantity to form the impermeable core that typically seals this type of dam.

The solution came from technologies that have been used for decades in countries like Norway and Sweden, based on… hydraulic asphalt core.

About 76 thousand cubic meters Asphalt mixtures heated to 150 degrees Celsius were produced on-site and applied in alternating layers with gravel and rock.

The goal is to create a waterproof core, flexible enough to adapt to thermal expansion without cracking., surrounded by millions of cubic meters of rockfill that absorb the water pressure.

Before that, it was necessary to erect a temporary dam, keep the valley dry, remove more than one million cubic meters of soil and hundreds of thousands of cubic meters of rock, apply a concrete base, and inject liquid cement into natural fissures, forming a curtain that reduces seepage into the rock mass.

Tunnels, giant pipes, and the path of the water to the new lake.

The dam only makes sense within a much larger system.

The water that supplies Chimney Hollow does not originate in the flooded valley, but flows through it. an underground network of tunnels and pipes considered the artificial circulatory system of Colorado.

The route begins at the Windy Gap reservoir, passes through pumping stations towards Granby Lake, and continues through other bodies of water before entering the… Adams Tunnel, approximately 21 kilometers long, carved directly into the Rocky Mountains.

Next, the water passes through pressure control valves and travels through steel pipes with a diameter greater than 3 meters until it reaches the Chimney Hollow reservoir.

This entire system is encased in concrete and monitored by sensors to reduce losses along the way.

The filling of the lake should occur slowly, over approximately three years, depending on snowmelt and more generous precipitation cycles.

Artificial intelligence, sensors, and the safety of a 100-meter dam.

In addition to erecting a A wall nearly 100 meters high at the bottom of a valley.The project incorporated a digital layer for continuous monitoring.

More than 500 seismic and pressure sensors They were installed deep within the asphalt core and along the structure, recording everything from variations in water level to minute vibrations caused by wind, bombs, and small earthquakes tens of kilometers away.

The data is sent to the Northern Water control center, where Artificial intelligence algorithms learn normal patterns of dam behavior. and look for signs of anomalies that indicate infiltrations, displacements or deformations.

Autonomous drones regularly fly over the slope, generating three-dimensional maps centimeter by centimeter and helping to identify any surface changes.

The structure was designed to withstand significant seismic activity and operates with safety margins that account for evaporation, sediment, and abrupt load variations, typical of reservoirs subject to more intense filling and emptying cycles.

Submerged valley, compensatory reforestation, and environmental criticism.

Before the construction, the Chimney Hollow valley was home to Young pine trees, streams, camping areas, and wildlife typical of the Colorado mountains..

With the dam closed, this entire landscape was submerged under dozens of meters of water.

For many residents, this is an irreversible loss linked to childhood memories and the recreational use of the area.

In return, the state government and project managers promised reforest an area equivalent to twice the original size., planting two new trees for each tree lost and creating biological corridors around the lake to allow animals to continue circulating.

Part of the funding comes from funds aimed at protecting aquatic birds and small native mammals.

Hydrologists warn of the risk of a 15 to 20 percent reduction in downstream flow on the Big Thompson River During dry periods, this threatens wetlands and agricultural systems that support tens of thousands of people in the Platte River basin.

The debate pits the need for urban water security against the preservation of ecosystems that depend on the natural flow of rivers.

Colorado’s new water weapon against decades of drought.

In full operation, Chimney Hollow should function as Colorado water memory: storing water during the winter and spring, during the thaw, and releasing flows in the summer and autumn, when agricultural and urban demand peaks.

Belts of vegetation around the lake, IoT sensors, and the combined use of satellites and drones are being used to try to control evaporation, monitor water quality, and track sediment movement.

For proponents of the project, the A wall nearly 100 meters high at the bottom of a valley. This is the price to pay for a system capable of guaranteeing drinking water to half a million people in a more unstable climate scenario.

For critics, the dam symbolizes the insistence on large-scale solutions that postpone deeper debates about consumption. Waste and land use patterns in arid regions.

Given this dam that submerges an entire valley to create a strategic reservoir, do you think that erecting a wall almost 100 meters high at the bottom of a valley is a necessary response to drought or too high an environmental cost for the future?