By Karl Plume
(Reuters) - The drought-drained Mississippi River will rise slightly later this week along a stretch between St. Louis and Cairo, Illinois, but will then continue its decline toward historic lows, according to a National Weather Service forecast.
Low water due to the worst U.S. drought since 1956 has already impeded the flow of billions of dollars in grain, coal, fertilizer and other commodities between the central United States and export terminals at the Gulf of Mexico.
A further drop in river levels could halt commercial shipping traffic entirely as soon as this weekend, the American Waterways Operators and the Waterways Council Inc said in a statement on Wednesday.
Last week, the council said the river along the Cairo-St. Louis stretch would be too low for navigation by January 7 but on Wednesday said shipping may come to a halt between January 5 and 15.
A shutdown could affect more than 8,000 jobs, cost $54 million in wages and benefits, and halt the movement of 7.2 million tons of commodities valued at $2.8 billion, the two industry groups said.
Prime areas of concern are near the Illinois towns of Thebes and Grand Tower, where river-bottom rock pinnacles pose a risk to boats when the river is exceptionally low.
"That reach has historically been the rock-bottom part of the river and pretty much everywhere else on the mid-Mississippi is sand- and silt-bottom," said U.S. Army Corps of Engineers spokesman Mike Petersen.
The Corps has been dredging various soft-bottom sections of the river nearly round-the-clock for months to maintain a deep enough shipping channel, but a project to remove the most threatening rock pinnacles will not be completed until the end of January.
The vast majority of commercial vessels need a depth of at least nine feet so shippers are closely monitoring river gauges and forecasts.
The Mississippi River gauge at Thebes fell from a reading of 4.45 feet late last week to 4.1 feet at midday on Wednesday. It was forecast to rise to 4.2 feet on Friday morning before slipping to 3.7 feet by Sunday, the lowest level at Thebes since 1988 and the second lowest on record.
Gauge readings do not reflect the actual depth of the river at a certain location because the gauges are fixed and the river's bottom is steadily changing with the current. But they do aid navigation as a shorter term reference point.
The Army Corps has said once the Thebes gauge reads 2 feet, boats with a nine-foot draft, or distance between the water's surface and the lowest point of the vessel, would be at risk of hitting rock pinnacles there.
"We lose 9 feet of depth for the navigation at about 2 feet on the Thebes gauge. That's when those rocks become an issue," Petersen said.
(Reporting by Karl Plume in Chicago; Editing by Steve Orlofsky)
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