Denver Water draining Antero Reservoir to save water during historic drought in Colorado

Antero Reservoir is being drained almost completely dry this spring, as Denver Water moves roughly 18,000 acre feet of water downstream to Cheesman Reservoir in an effort to preserve water during one of the driest years Colorado has seen in decades.

The move comes after what Denver Water says was the lowest peak snowpack and earliest snowmelt on record in Colorado's mountains. According to Denver Water Manager of Water Supply Nathan Elder, Antero's design makes it especially vulnerable during hot, dry years.

"It has a large surface area, it's very windy up here, and the reservoir is also really shallow," Elder said. "So those things contribute to a really high evaporation rate."

Denver Water estimated nearly 25% of the reservoir water would have evaporated over the rest of the year if it stayed in Antero. By transferring the water downstream into deeper reservoirs like Cheesman, the utility says it can save about 5,000-acre feet of water from evaporating. For reference, one acre foot of water is enough to supply roughly three to four homes for an entire year.

"We'll be moving about 18,000-acre feet from Antero Reservoir downstream through Spinney Reservoir, through Eleven Mile Reservoir, and then eventually into Cheesman Reservoir," Elder explained.

Denver Water said the tough decision to close the popular recreation site for boaters and anglers comes from a very dry Colorado environment, worse than the infamous 2002 drought.

"We had the lowest peak snowpack we've ever seen," Elder said. "We had the earliest melt off that we've ever seen."

Typically, Antero receives around 13,700-acre feet of inflow annually. This year, Denver Water said it could receive a total as little as 500-acre feet. The utility also said Colorado just experienced the warmest water year recorded in the 131 years of snowpack measurements that have been tracked statewide.

"The worst part was the heat wave we experienced in March," Elder said. "It really took a big hit to our snowpack."

Denver Water said about 90% of its entire supply depends on snowpack runoff between April and July.

The draining operation began on May 1. Normally, Denver Water releases around 5 to 10 cubic feet per second (cfs) from Antero downstream to feed the rest of the system. Right now, they are releasing about 250 cfs to get the water out before it loses any more water to evaporation. Officials said only around 20-acre feet of water will remain once the process is complete, leaving the reservoir effectively empty.

The reservoir is now closed to the public for the foreseeable future, impacting boating, fishing, and tourism around the South Park area. As for when it will refill, there is no estimate from Denver Water at this point; it all depends on how much snow we get next year, considering nothing fills into this reservoir.

"2002, when we drained it that year, it took about four years to fully refill," Elder said. "In 2015, it only took a year to refill."

As for the critters who live around the reservoir, Colorado Parks and Wildlife plans to conduct fish salvage operations as water levels continue dropping.

"Those fish will be moved downstream to Spinney Mountain Reservoir and Eleven Mile Reservoir," Elder said.

As for what it means to customers who are running off of Denver Water's system, Elder said they should expect stricter watering rules as drought conditions worsen.

"I'd really like our customers to take away just how serious the situation is," Elder said. "By following those watering rules that we have in place every year, following the restrictions of this year, you can really limit the impacts."

Elder said draining Antero was not a decision the utility made lightly.

"We're going to see a lot of hard decisions this year," he said. "Whether it's draining a reservoir, whether it's a farmer figuring out how much of their fields that they want to plant ... it's going to be a very tough year."