Robins, check; baseball Opening Day, check; first towboats to arrive in St. Paul on the Mississippi River, check — finally.

The arrival — and brief stay — of the Angela K, with its tow of 12 barges, is another eagerly anticipated sign of spring, not to mention one with real economic importance, signaling the launch of the navigation season on the Upper Mississippi River.

The Angela K passed through Lock and Dam No. 2 at Hastings on Wednesday bound for St. Paul, which the U.S. Army Corps of Engineers considers the unofficial starting point for the navigation season, said Patrick Moes, spokesman for the U.S. Army Corps of Engineers district headquarters in St. Paul.

After dropping off its load of fertilizer and cottonseed, the Angela K turned about and headed back down the river on Thursday.

This year's start of the navigation season was the latest on the Upper Mississippi since 1970, with the exception of the 2001 season, which was disrupted by major flooding. The earliest date for a northbound tow to reach Lock and Dam No. 2 was March 4, which happened in 1983, 1984 and 2000. The average date is March 22.

The starting date from Lock and Dam No. 2 is also more than trivia. Traders of grain and other commodities calculate freight rates based on their ability to supply barges at certain dates pegged to the first lockage time at Hastings.

The navigation season was delayed this year due to historically thick ice on Lake Pepin, south of Red Wing, Moes said. Lake Pepin is the last part of the river to see ice break up, because the river is wider there and subsequently the current is slower than at other reaches of the river.

North of Lake Pepin, local towboats have been working at the three Twin Cities local dams since April 2. South of the lake, in Winona, the shipping season has been open for a couple of weeks.

A blend of warm temperatures, rain and driving winds helped speed the ice break up, Moes said. The last measurement of the ice was taken on April 9, when the thickest point was 22 inches. When the Corps did a visual inspection five days later, the ice had melted.

Twitter: @StribJAnderson