ST. LOUIS — Water levels on the drought-plagued Mississippi River are expected to keep dropping over the next several weeks, according to a new forecast Wednesday that comes amid worries that barge traffic soon could be squeezed along a key stretch of the vital shipping corridor.

The latest outlook by National Weather Service hydrologists shows the river at St. Louis falling to about 9 feet deep by Dec. 30 — a day later than earlier predicted — and by a half foot more by Jan. 9, barring significant rainfall. The Coast Guard has said further restrictions on barge traffic are likely if the river’s depth dips to 9 feet.

The river depth in St. Louis as of Wednesday was about 12 feet.

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 was worsened last month when the Army Corps of Engineers cut the outflow from an upper Missouri River dam by two-thirds, meaning far less Missouri River water 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.

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.

Complicating matters are two rock pinnacles impinging on barge traffic on the river south of St. Louis.

U.S. Sen. Roy Blunt, a Missouri Republican, said Tuesday the corps could have crews beginning to blast the pinnacles as early as next week — two months ahead of schedule — in what would be an important step in helping keep the shipping channel open, though it remains unclear how long such work might take.

The corps hasn’t publicly discussed the matter this week, and a national spokesman for the agency told The Associated Press on Wednesday he was trying to sort out the timing of the pinnacle-removal effort.

The corps removed many rock pinnacles in the southern Illinois area more than two decades ago. Sonar wasn’t as advanced in the late 1980s and new technology recently revealed formations the agency missed near the southern Illinois towns of Thebes and Grand Tower.

The Coast Guard has said it does not expect to close the river. But Tom Allegretti, president and CEO of American Waterways Operators trade group, said any additional restrictions on barges will leave the river “as good as closed.”

“This is still very much a crisis situation,” said the group’s Ann McCulloch.