Dillon Reservoir Freezes Over - On Schedule

Dillon Reservoir is frozen. The average date of ice-over, as the Denver Water dam keepers call it, has held steady at Dec. 25 for the past 40 years, with no visible trend toward earlier or later.

But that doesn't hold true for lake ice in general, according to the Alaska Ice and Snow Observatory Network, specializing in studying lake ice. According to the researchers' web site, "the duration of ... ice covers has decreased in response to a general increase in air temperatures."

This year, the ice on Dillon Reservoir formed on the night between Dec. 22 and Dec. 23, just a couple of days ahead of schedule. By the evening of Dec. 23, the ice was already a couple of inches thick near the Dillon Marina.

The earliest the lake has ever frozen was Dec. 10, back in 1972, but last year wasn't far behind. In 2005, the ice formed on Dec. 11, the second-earliest date ever. Records from last year don't show particularly cold temperatures in late November or early December. In fact, the average daily minimum temperature for Nov. 2005 was 12.6 degrees, well above the historic average of 9.7 degrees. But the average high temperature for Dec. 2005 was quite a bit colder than the norm, at 26.4 degrees, compared to the historic average of 32.8 degrees.

The latest date for the lake to freeze up was Jan. 31, in 1981. That late date was a real anomaly in the records. There have been only four other years since 1966 when it took until January for the ice to form, and all those ice-over dates came early - within the first few days of the month.

If there is a slight trend to be spotted in the graphs and figures provided by Denver Water, it's the melt-out dates, which does seem to have occurred slightly earlier during the past 10 years or so. Since 1996, the melt-out date has been earlier than the long-term historic average.The earliest ice-melt on record is April 28, 2002. The latest date on record is May 31, in 1982.

To learn more about lake ice research, visit the Alaska Ice and Snow Observatory Network online at www.gi.alaska.edu/alison/ALISON_SCIENCE_Lake.html. The web site includes cool ice photos, tons of basic ice physics, a basic introduction to the laws of thermodynamics that govern ice formation, and even an interactive quiz on ice.