By this time of year, with spring tantalizingly close yet stubbornly elusive, longer days and roosting robins signal a gradual release of winter’s hold.
And on Lake Pepin southeast of Red Wing, Minn., the U.S. Army Corps of Engineers is watching, and measuring, the ice.
In an annual ritual closely watched by river shippers, the corps last month began gauging ice depths at nearly two dozen points along the 21-mile-long lake, the largest on the Mississippi River. Above and below the lake, which has an average width of about 1 ½ miles, faster river currents in the narrower channel keep the water open. That means the frozen lake is the last obstacle to northbound towboats and their barge shipments to ports that include Minneapolis, St. Paul and Savage.
The verdict: This year the shipping season, and by unofficial extension, spring, might take a little longer to arrive than average. For the past three decades, spring shipping typically has begun on March 20. But with the ice averaging 2 feet deep in most spots, it will take a mighty thaw to hit that mark, said Patrick Moes, spokesman for the corps.
“When it comes to the actual start of the navigation season, it’s really hard to say,” he said. Ice depths need to be about 8 to 12 inches before they are thin enough for the stout towboats to muscle their way through the lake.
Last year, the first towboat came through on St. Patrick’s Day, when Twin Cities temperatures hit a record 80 degrees. “That was more of an anomaly,” Moes said. According to corps records, the earliest tow arrival in St. Paul was on March 4 in both 1984 and 2000. The latest arrival on record was May 11, 2001, due to floodwaters.
The process for measuring the ice is pretty basic, Moes explained. Two-person survey crews (a third follows along on land with a truck and trailer) from the corps’ Fountain City Service Base in Fountain City, Wis., use an airboat — the type commonly seen in the Everglades — and a global positioning system to identify the same precise measurement locations from where the lake begins at the mouth of the Chippewa River to near Red Wing.
Using an auger, they drill down through the ice pack. Crews are looking for two types of ice, blue and white, he said. White ice is the result of thawing and freezing cycles, and as a result holds a lot of air bubbles that make it softer. Blue (or black) ice is compressed and solid, and harder to break through.
On Wednesday, the most recent measuring day, white ice averaged 2 inches in most spots, blue ice 22 inches. A few measuring sites were clear of ice.
The data is posted online (www.tinyurl.com/ajy6fnt), so shippers who use the Mississippi to transport commodities can decide when to get the new season rolling.