By Karl Plume

(Reuters) - Commercial barge traffic on a critical stretch of the Mississippi River south of St. Louis may be severely restricted or halted entirely next month as drought conservation measures stem the inflow of water from the Missouri River, government and industry sources said on Thursday.

With the worst U.S. drought in 56 years sapping the reservoirs that feed the Missouri River, the flow of water from Gavins Point Dam, the system's southernmost dam near Yankton, South Dakota, will be reduced beginning in the third week of November, the Army Corps of Engineers said.

"In September we had the lowest inflow month that we've seen in our system since we began keeping detailed records in 1898. We only got about 25 percent of normal inflows," said Monique Farmer, spokeswoman for the Army Corps of Engineers Northwestern division.

"When we did our September 1 storage check we saw that water levels had decreased (from midsummer) and we made the decision that we had to take drought conservation measures during the winter," she said.

The Army Corps will incrementally reduce water released from Gavins Point Dam to about 12,000 cubic feet per second and maintain that flow through the winter to conserve water in the reservoir system. The water released through the dam during the winter is normally between 12,000 and 17,000 cubic feet per second, Farmer said.

MISSOURI FEEDS MISSISSIPPI

The Missouri River normally accounts for about 60 percent of the water in the Mississippi River between St. Louis and Cairo, Illinois, a key stretch of the major grain shipping waterway that connects farms in the Midwest with export terminals at the Gulf Coast. Gasoline, coal and other commodities are also shipped through the area.

However, water on the Mississippi River was also reduced by the drought so the Missouri River accounted for an unusually high 79 percent of the flow in that stretch this season, said Michael Toohey, president and CEO of the Waterways Council, a public policy organization representing ports and shippers.

"If those flows are reduced, navigation may be interrupted on or around December 10," he said.

The Army Corps has been dredging the Mississippi almost nonstop for months to maintain a navigable channel, but scores of barges and boats have still run aground, snarling river traffic at times.

"The critical area is between St. Louis and Cairo, where we transition from locked river to open river and before we get all that water from the Ohio," said Mike Petersen, spokesman for the Army Corps' St. Louis district. Continued...