The air temperature was 5 degrees above zero. The wind chill didn’t matter, because once Bill Chelmowski and Dan DeVaney climbed onto the airboat, they created more wind chill than nature did.
The two are members of the U.S. Army Corps of Engineers crew that conducts the ice depth surveys on Lake Pepin every February and March.
“When we do the initial survey every season, we put in stakes and take measurements at every mile marker on the lake,” said Chelmowski. “That takes us about four hours.”
They set the markers on a line that follows the commercial navigation route from Mile 765 to Mile 786 on the Upper Mississippi River, because Lake Pepin is the only place that the river slows down enough to freeze completely across.
“This year we have about 16 inches of ice,” Chelmowski said. “Two years ago we had twice that much. We average about 22-24 inches on most of Lake Pepin.”
He said the thickness of the ice can vary significantly around the lake, noting that the ice was 16 inches at Old Frontenac, but very thin from Point No Point and upstream.
Patrick Moes, public affairs specialist with the Army Corps of Engineers, St. Paul District, said there are many variables that determine how thick the ice will be at any time during the season.
“It takes weeks for the ice to form on Lake Pepin,” he said. “When you get sub-zero temperatures, you can build up ice pretty quickly, but this has been a mild winter until now. This week, with the polar vortex, is the first time we’ve had sustained cold temperatures this winter.”
The ice surveys are done every two weeks until the ice starts to melt. Then the crew measures every week as they watch for ice that is 12 inches or less, the mark when the shipping industry starts to think about moving commodities up and down the river.
“They have to make a decision based on when it is safe for their employees, first and foremost, and then to be sure that their equipment isn’t going to be damaged,” Moes said. “The ice can be brutal on equipment. Once Pepin is open, they have full run of the river all the way up to St. Paul.”
The melting of the ice on Lake Pepin can start even if the air temperature is not above 32 degrees, Moes explained.
“We get the sun on the top and the water flowing underneath, and the friction of the water starts the melting,” he said. “When it gets going, the ice can be here one day and gone the next.”
Moes said that the lower snow amounts this year might help with an early start to the commercial navigation season, but he noted that Lock and Dam 4 near Alma, Wis., is currently drained for repair work.
“We are pretty optimistic that once Lock and Dam 4 is done with winter maintenance, we should be good to go,” he said.
More information about the ice surveys and the latest depth measurements can be found on the Army Corps of Engineers’ website at www.mvp.usace.army.mil/Missions/Navigation/Ice-Measurements.