Front Range reservoir to store some of Lake Granby’s water

The public enjoys the water on Lake Granby on June 13, 2020. The Grand County reservoir, which will be lower than average this winter, could see future impacts from the building of the Chimney Hollow Reservoir in northeastern Colorado. The new reservoir will be filled with water from the Colorado River Headwaters.
Amy Golden/Sky-Hi News

Northern Water’s newest reservoir could draw down Lake Granby’s levels in the coming years.

The water company broke ground in August on the Chimney Hollow Reservoir, which will be built west of Carter Lake in Larimer County. The water that fills the reservoir will come from the Colorado River Headwaters in Grand County.

Northern Water Public Information Officer Jeff Stahla said there would be “some impacts” to Lake Granby water levels with the addition of Chimney Hollow that should be complete by 2029.

It took 20 years to get the permits aligned for the new reservoir, which will rise 350 feet off the dry valley floor. Northern Water said the dam that forms Chimney Hollow should add resilience to the water supply for more than 500,000 northeastern Colorado residents.

The Chimney Hollow Reservoir is meant to give participants in the Windy Gap Firming Project — Broomfield, Platte River Power Authority, Loveland, Greeley, Longmont, Erie, Little Thompson Water District, Superior, Louisville, Fort Lupton, Lafayette and Center Weld County Water District — a consistent water supply during dry years.

Since the Windy Gap Project was envisioned, water managers identified the need for additional storage dedicated to Windy Gap water.

Right now, the Windy Gap Project uses Lake Granby to store water. Windy Gap sometimes has more water than it can hold and excess amounts are held in Lake Granby. However, Lake Granby’s first priority is to store Colorado-Big Thompson Project water, which can leave Windy Gap’s excess water running down river.

“While the Windy Gap Project has been delivering water in many years, there have been some years in which it couldn’t deliver because there was no room in Lake Granby to put the water,” Stahla said. “The addition of Chimney Hollow Reservoir will mean there will be more years in which there is space available in Lake Granby to store Windy Gap water, which means it’s probable there will be fewer years in which Lake Granby spills over the spillway.”

With the new reservoir, Northern Water has promised to support a number of environmental efforts on the headwaters of the Colorado River in Grand County. That includes the construction of the Colorado River Connectivity Channel, wastewater treatment plant upgrades in the Fraser Valley and environmental improvement projects with the Learning By Doing coalition.

With the reservoir a few years off from being filled, Lake Granby’s current levels have to do with other environmental factors. As of last week, the reservoir was at an elevation of 8,259 feet, according to Stahla. Northern Water is forecasting that Lake Granby will fall to 8,241 feet this April.

That will be the lowest the lake has been since 2013, when Lake Granby dropped to 8,217 feet.

As for why Lake Granby is dropping to a lower-than-normal level, Stahla said that has to do with the low soil moisture going into the 2020-21 winter season. While snowpack was near average, a larger share of water from the snow went into the soil and not into the tributaries that feed the Colorado River and Lake Granby.

Stahla added that the past several years have seen Lake Granby at above-average lake levels compared to historical numbers. In 2019, Lake Granby dropped down to 8,251 feet, just 10 feet more than the current forecast. The lowest the water level has been in recent history was 8,191 feet, which was on March 17, 2003.

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