Anyone living near a pond or lake may have the good fortune to observe the ice-out phenomenon. Although some people believe that ice sinks when it leaves a lake surface, that is not true. Ice is lighter than water.

The process starts with ice retreating from the shore, creating a belt of open water. Then the band widens as the rest of the ice cover turns dark.

With more warm days the main ice sheet weakens and begins to fracture in large sections, some moved by wind. This is followed by melting of ice crystals on the edges of the floating sections that contact warm water and air. Wind will sweep the last ice sheets from the lake. The remaining ice is in chunks up to 7 inches thick and honeycombed as sheets are pushed ashore.

I consider ice-out to have occurred when at least 90% of the lake is free of ice.

Ice-out is a wonderful spring sign, and as ice is leaving, common loons, great blue herons and ducks and other waterfowl return or stop off on their migration routes.

There are thousands of citizen observers reporting data in Minnesota. They collect information on monarch butterflies, autumn leaf colors, bird migrations and nesting and water quality. They do frog surveys, watch for storms, measure precipitation and more. Their observations help the Department of Natural Resources, academic researchers and others to understand the pulse of our planet and build our state's knowledge records.

If you would like to report a 2021 ice-out date for a northern Minnesota lake, please note the date when 90% or more of the ice is out and include the name of the lake, county and nearest city or town. Report to MN State Climatology Office on Facebook or e-mail

This list of median ice-outs for five northern lakes will help give perspective to what happens this year compared to average ice-outs. Median ice-outs: Lake Mille Lacs, April 25; Lake Bemidji, April 24; Leech Lake, April 28; Lake Vermilion, April 30; and Lake of the Woods, May 5.

Jim Gilbert taught and worked as a naturalist for 50 years.