Ice not expected to affect river navigation here

A blast of Artic air pushing into the Midwest has not only brought single-digit temperatures to Southeast Missouri, but has also dropped the Mississippi River water temperature to the low 30s.

According to the U.S. Geological Survey (USGS), the water temperature at Cape Girardeau has fallen steadily over the past week and by Friday had reached 33 degrees Fahrenheit with expectations of going lower.

Water freezes at 32 degrees, so that means the river at Cape Girardeau will turn to ice, right?

Not necessarily.

"Although water freezes at 32 degrees, flowing water needs to be colder than that to freeze into ice," Gary Johnson, a hydrologist with the USGS told the Southeast Missourian.

Johnson is the operations coordinator at the USGS's Central Midwest Water Science Center in Urbana, Illinois. River currents below the surface, he said, have inconsistent temperatures and don't necessarily match surface temperatures.

"Large rivers rarely freeze throughout because while water is flowing, its potential energy is constantly being converted to heat energy that resists freezing on the molecular level and subsequent crystallization," Johnson said. "For flowing water to freeze, the temperature has to be much colder than 32 degrees."

Farther north, in the St. Louis vicinity, there have been reports of ice accumulations in the river.

"We've got ice at locks 24 and 25 and the Alton (Illinois) lock and dam is holding back quite a bit of ice," said Janet Merideth, public affairs specialist with the U.S. Army Corps of Engineers district office in St. Louis. "There is also some south of Alton."

She described that ice as "pancake" and said for the most part is has not interfered with river navigation and that it would take an "extended period of extremely low temperatures" for enough ice to form to the point that it would become a navigational hazard.

However, a sufficient accumulation of ice, she said, can interfere with lock and dam gate operations, and some ice could travel downstream and possibly make its way to the Cape Girardeau riverfront.

Some of that ice could accumulate along the riverbank, but Merideth cautioned against stepping on any of it no matter how solid it might appear.

"No ice is safe," she said. "There is always current running underneath it."

As of Friday afternoon, the river level on the Cape Girardeau riverfront measured 13.3 feet. The National Weather Service office in Paducah, Kentucky, is forecasting a drop to about 10 feet by Feb. 24. That would be about a foot higher than the 9.03 feet recorded on the Cape Girardeau gauge Feb. 10, 2018, which was the lowest level recorded on the Cape Girardeau gauge in the last three years.

The Corps of Engineers' St. Louis district is responsible for maintaining a navigation channel along the Mississippi between Hannibal, Missouri, and Cairo, Illinois, that is at least 300 feet wide and a minimum of 9 feet deep to accommodate barge traffic, which often has an 8-foot draft.

However, the gauge reading does not always reflect the depth of the navigation channel, which is often deeper than the gauge measurements.