When did the Dillon Reservoir freeze over? Later than usual, according to Denver Water.
In the winter, the Dillon Reservoir also provides water for snowmaking at Arapahoe Basin Ski Area, Breckenridge Ski Resort, Copper Mountain, Keystone Resort, Winter Park Resort and the Frisco Adventure Park

Andrew Maciejewski/Summit Daily News
Editor’s note: This story has been updated to correct the date attributed to the earliest recorded time that Dillon Reservoir fully froze over.
The Dillon Reservoir froze over this month on Jan. 9, later than average, according to Denver Water.
Denver Water spokesperson Jimmy Luthye said in an email that on average the Dillon Reservoir freezes over on Dec. 25, Christmas Day. When the reservoir freezes over in the winter, it becomes a playground for recreational activities, from cross-country skiing to ice fishing, ice hockey, dogsled racing, kiteboarding and more.
“A number of factors lead to the reservoir freezing over, including cold temperatures, the current water elevation of the reservoir (if the reservoir is less full, it tends to freeze sooner), limited wind and cloud cover,” Luthye said.
With a capacity of 257,300 acre-feet and almost 27 miles of shoreline, the Dillon Reservoir diverts water under the Continental Divide through the Harold D. Roberts Tunnel, providing a source of drinking water to Denver and the Front Range. The reservoir is currently about 80% full.
Denver Water began keeping records on when the reservoir froze over beginning in 1966, about three years after the dam was created. In those 59 years of record-keeping at the reservoir, this is only the 11th time that the man-made body of water has frozen over later than Jan. 1, Luthye said. The latest the reservoir has ever frozen over was January 31, 1981, while the earliest it ever froze over was Dec. 10, 1972, he said.
Ahead of each winter, a giant plug is installed in Dillon Reservoir’s “Morning Glory” spillway, which measures 15 feet in diameter and features a more than 200-foot drop. The 6-ton steel plug prevents cold air that could create ice build ups from entering the spillway. It also prevents chunks of ice and snow from falling into the spillway and crashing more than 200 feet below, according to Denver Water.
In the winter, the Colorado Division of Water Resources Dam Safety Program requires the reservoir to be 3 feet from full due to the design of the spillway, helping to prevent ice damage, Luthye said.
Denver Water also allows six ski areas to borrow up to 6,300 acre-feet of water — or about 2 billion gallons — for snowmaking each ski season. Arapahoe Basin Ski Area, Breckenridge Ski Resort, Copper Mountain, Keystone Resort, Winter Park Resort and the Frisco Adventure Park all tap into the reservoir’s resources for snowmaking in the winter, according to Denver Water. At the end of the season, the snow that was made at Summit County ski resorts melts and runs off back into the reservoir.
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At Keystone Resort, the Roberts Tunnel, which runs underneath the reservoir, connecting it to the Front Range, runs beneath the resort, Luthye said. That allows Keystone to pump water up from the tunnel for snowmaking, he said.
In the winter the town of Dillon also grooms lake loops into the frozen reservoir allowing locals and visitors to cross country ski and recreate on the ice. The town also hosts events on the reservoir most years, including ice hockey tournaments and full moon parties.

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