If bodies of water are any indication, spring in Minnesota might be around the corner.

An unusually warm winter helped launch the unofficial navigation season on the Mississippi River Thursday, and local experts are expecting a potential record-breaking ice-out date for Lake Minnetonka this year.

A spate of other metro-area lakes are also setting records, according to the Department of Natural Resources, including Lake Nokomis, Lake Harriet and Lake Calhoun where ice-outs were declared Tuesday.

"There's a lot of record early ice-outs this year," said Steve Woods, executive director of the Freshwater Society, a nonprofit that's tracked ice-out on Minnetonka since 1855.

On Thursday morning, the first tow boat of the 2017 Mississippi navigation season reached Lock and Dam 2 in Hastings. The vessel then made its way to St. Paul. The U.S. Army Corps of Engineers considers the first tow to arrive at Lock and Dam 2 as the unofficial start of the navigation season.

The earliest opening date was March 4, in 1983, 1984 and 2000. Thursday's unofficial opening of navigation season comes a little less than two weeks ahead of the March 22 average.

Lake Calhoun and other city lakes had ice-out declared on Thursday.

"It's not extremely unusual, but it's earlier than the norm," said George Stringham, a corps spokesman.

An early navigation start could mean a longer shipping season depending on when the river freezes again in the fall, Stringham said. But that doesn't necessarily mean that shipping companies will move more goods.

The commercial navigation industry estimates an annual average savings of nearly $270 million by using the inland waterways instead of shipping via land, according to the corps.

On Minnetonka, this year's ice-out could be the second earliest in the lake's history, thanks to recent rain and high winds, though the rate of ice loss will likely slow due to lower than normal temperatures predicted for this weekend. The record Minnetonka ice-out was set March 11, 1878, and the second-earliest was notched last year on March 17.

Last year's ice-out on the 14,500-acre lake helped cement a yearslong trend of earlier and earlier ice-outs. While the median date is April 14, more than half the lake's ice-outs in the past 20 years have occurred before that date.

Early ice-melt times can affect lake temperatures, resulting in changes to fish populations. Woods said less ice cover can also cause lakes to lose more water to evaporation.

There's no universal method for determining ice-out in Minnesota. Decades ago, for instance, residents would put a car on the lake and mark ice-out by the date when it fell through. But the trends are significant as long as methods are consistent from year to year, Woods said.

Ice-out on Lake Minnetonka is decided by Hennepin County deputies along with the Water Patrol and the Freshwater Society.

They declare ice-out not when the lake is completely ice-free but when a boat can navigate through all its channels and bays without being stopped by ice floes.

To check other lakes in Minnesota, go to dnr.state.mn.us/ice_out.

Haley Hansen is a University of Minnesota student on assignment for the Star Tribune.

Kelly Smith • 612-673-4141