ST. LOUIS — The Army Corps of Engineers said Friday it was finalizing how soon crews could begin urgently clearing submerged rock formations that are hindering Mississippi River barges as concerns linger that the drought-plagued waterway soon may be closed to shipping.

The corps has hired Newt Marine Service of Dubuque, Iowa, and Kokosing Construction Co. of Fredericktown, Ohio, to rid a six-mile stretch of the river of rock pinnacles south of St. Louis near Thebes, Ill. — an effort some U.S. lawmakers suggested in recent days could begin as early as next week.

But the corps again held off on confirming any time frame, saying Friday it was sorting out with the contractors how swiftly they can get to the work site, prep it and begin blasting away the granite. Underscoring the project’s pressing nature, the corps — at the behest of federal lawmakers from Mississippi River states — has moved up the work from what had been a projected February start.

“We expect work to begin very soon, but we don’t have a definite date,” said Mike Petersen, a corps spokesman in St. Louis. “It’s hard to work down until we have (the contractors’) work plan.”

Months of drought have left water levels up to 20 feet below normal along a 180-mile stretch of the river from St. Louis to Cairo, Ill. The problem worsened last month when the corps cut the outflow from an upper Missouri River dam by two-thirds, meaning far less water from the Missouri River flows into the Mississippi.

Barges on the Mississippi already are carrying lighter and more frequent loads, and some operators said they’ll halt shipping if they face more restrictions from reduced water levels.

National Weather Service hydrologists forecast that the river, barring significant rainfall, at St. Louis could fall by the end of this month to about 9 feet deep — the point at which the Coast Guard has said further restrictions on barge traffic are likely. The river depth in St. Louis as of Friday was about 11 feet.

Barge industry trade groups said a prolonged stoppage of shipping on the Mississippi could have an economic impact reaching into the billions of dollars, with the movement of agricultural products, coal, petroleum and other goods reliant on river for transit.

Bob Anderson, a corps spokesman in Vicksburg, Miss., said the blasting involves removing 890 cubic yards of very dense granite — roughly enough to fill 50 dump trucks — that typically would be beneath sand on the river bottom but has been exposed by the corps’ dredging efforts to keep the channel open.

“We’ve dredged so far and so deep,” taking out 6 million cubic yards of sand and sediment between St. Louis and Cairo, that the rock formations now come into play, Anderson said.

The blasting and rock removal, tentatively expected to cost about $8 million, could bottleneck barge traffic in and near the work zone. That stretch of river would be closed for as many as 16 hours a day to ensure the safety of the shippers, Anderson said.