ST. LOUIS (AP) — 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 6-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.