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Where does your water come from?

Denver Water’s 4,000-square-mile collection area supports 1.5 million people across the metro area.

Denver Water, which provides water to 1.5 million people across the metro area, relies on a system that collects rain and snow from across 4,000 square miles of mountains and foothills west of Denver.

“Every year it’s helpful to have a large and diverse water collection system, because you never know how much snow we’ll see in Colorado and where it will fall,” said Nathan Elder, water supply manager at Denver Water. 

On an average year, the utility captures 290,000 acre-feet of rain and snowmelt in its collection system. That’s roughly 94 billion gallons of water — or enough to fill up nearly 157 Empower Fields at Mile High. 

The water flows down rivers and streams, then through a network of tunnels, pipelines and canals to treatment facilities in the Front Range to be cleaned for delivery to homes and businesses. Because most of the water comes from mountain snowmelt in the spring, water is stored in mountain reservoirs until it is needed.

Construction on the elaborate reservoir storage collection system began in the early 1900s when Cheesman Dam was built along the South Platte River southwest of Denver. Over the years, additional dams and tunnels were built to capture and store more water for Denver’s growing population. 

Having a large and diverse collection system was critical to Denver’s growth because the city lies in a semi-arid climate.

Denver Water’s collection system is centered on the state’s mountains and foothills, because that’s where most of Colorado’s precipitation falls. About 80% of Colorado’s snow and rain falls west of the Continental Divide. 

“The builders of the Denver Water collection system had to look to the mountains to ensure a reliable supply of water. There simply wasn’t enough moisture in the city to sustain the growing population,” Elder said. 

In Colorado, roughly 80% of the precipitation falls on the west side of the Continental Divide compared to 20% on the east side. This is why Front Range communities bring water over from the West Slope. Image credit: Denver Water.
Denver Water collects around 50% of its drinking water from tributaries of the Colorado River on the west side of the Continental Divide. The rest of the utility’s drinking water comes from the South Platte River Basin on the east side of the Continental Divide. Denver Water relies on roughly 2% of the water used in the state to support around 25% of Colorado’s population. Image credit: Denver Water.

Having a large geographic area to capture water is critical because it provides redundancy in times of drought, emergencies, infrastructure maintenance and improvement projects, according to Elder.

One example of the need for diverse sources of water occurred in 2020, when Denver Water had to close the Roberts Tunnel for maintenance. 

Closing the tunnel meant no water could be brought to customers from Dillon Reservoir in Summit County. Dillon is the largest reservoir in Denver Water’s system. The Roberts Tunnel brings water from Dillon under the Continental Divide to the Front Range.

With water in Dillon Reservoir out of reach until the tunnel’s maintenance project was finished, Denver Water had to rely on water coming down the South Platte River and from streams in Grand County. 

Another of Denver Water’s important water-carrying tunnels, the Moffat Tunnel, will be closed for maintenance in 2021, which means Denver Water won’t be able to access to water from Grand County during that project. 

The Moffat Tunnel carries water, collected from 36 Grand County streams, and funnels it under the Continental Divide to be stored in Gross Reservoir before going to treatment plants in the metro area.

In times of drought, if the South Platte River Basin is experiencing dry conditions but collection areas on the West Slope are experiencing normal or above average precipitation, Denver Water is able to draw water from rivers and streams on the west side of the Continental Divide.

Wildfires are another big concern for the utility. 

Eighty percent of Denver Water’s supply passes through Strontia Springs Reservoir, an area prone to wildfires like the 1996 Buffalo Creek and 2002 Hayman fires that impacted the reservoir and operations.

These factors are among the reasons Denver Water is planning to expand Gross Reservoir.

Denver Water’s storage capacity is imbalanced, with 90% of its storage capacity located in its South System and the remaining 10% located in its North System. 

The increased storage in Gross Reservoir would allow more water to be stored in Denver Water’s North Collection system provide additional redundancy in the utility’s ability to have a reliable water supply for customers. 

“Denver Water’s founders were not afraid to think big,” Elder said. “The foresight they had in the past greatly benefits Denver and the metro area now and will continue to help in the future.” 

The following maps break down the collection system and service area.

Denver Water’s collection area covers 4,000 square miles on both sides of the Continental Divide. On average, the utility captures around 290,000 acre-feet of water from rain and melting snow every year — that’s over 94 billion gallons. Image credit: Denver Water.
Denver Water stores water in 12 major reservoirs on both sides of the Continental Divide. Williams Fork and Wolford Mountain reservoirs are not used to store drinking water. Water collected and stored in those two reservoirs in the Colorado River Basin is released when needed to offset water that is diverted to the Front Range. Denver Water does not own Chatfield Reservoir, but stores water in it as part of an agreement with the U.S. Army Corps of Engineers. Image credit: Denver Water.
Denver Water’s drinking water collection area contains eight river basins. The majority of the water supply comes from the Blue, Fraser and South Platte river basins. Having multiple basins creates a large area to collect water. The large collection area is critical in having a reliable water supply because some river basins see more precipitation than others. Image credit: Denver Water.
Denver Water has two geographic areas where it collects water for its customers. The North System covers parts of Boulder, Clear Creek, Gilpin, Grand and Jefferson counties. The South System covers parts of Douglas, Jefferson, Park, Summit and Teller counties. Eighty percent of Denver Water’s supply comes from the South System while 20% comes from the North System. Image credit: Denver Water.
Denver Water’s North System spans parts of Boulder, Gilpin, Jefferson and Grand counties. Water collected from 36 streams in Grand County on the West Slope travels through the Moffat Tunnel under the Continental Divide. Water flows down South Boulder Creek and is either stored in Gross Reservoir or sent through a canal down to Ralston Reservoir and on to a Denver Water treatment plant. Image credit: Denver Water.
Denver Water’s South System captures water from the Blue and South Platte river basins. The Blue River is a tributary of the Colorado River on the West Slope of the state. Denver Water has rights to some of the water in the Blue River basin and stores it in Dillon Reservoir. From the reservoir, water can be delivered to the Front Range through the Roberts Tunnel under the Continental Divide. Water from the South Platte River is stored in a series of reservoirs running through Park, Douglas and Jefferson counties. Image credit: Denver Water. 
Denver Water has a water storage imbalance between its two collection systems with 90% of the reservoir storage in the South System compared to 10% in the North System. This storage imbalance creates vulnerability if there is a drought, mechanical issue or emergency that affects the South System. The storage imbalance is one of the reasons Denver Water is planning the Gross Reservoir Expansion Project. Image credit: Denver Water.
Denver Water provides water to more than 1.5 million people who live in the city and county of Denver and several suburbs. The utility serves water to around 25% of Colorado’s population with roughly 2% of the water used in the state. The service area will not expand as part of the 2013 Colorado River Cooperative Agreement. Image credit: Denver Water.