ST. LOUIS – The Army Corps of Engineers on Friday began reducing the flow from a Missouri River reservoir, a move expected to worsen low-water conditions on the Mississippi River and potentially bring barge traffic to a halt within weeks.

The Missouri flows into the Mississippi around a bend just north of St. Louis. One result of this year’s drought, the worst in decades, has been a big drop in water levels on both rivers.

The corps announced earlier this month that it would reduce the outflow from the Gavins Point Dam near Yankton, S.D., to protect the upper Missouri River basin. That drew an outcry from political leaders and businesses downstream, who warned that allowing the Mississippi to drop more could have devastating consequences.

Corps spokeswoman Monique Farmer told The Associated Press on Friday that the reduction began as scheduled that morning. By midday, the flow that had started at 37,500 cubic feet per second had been cut to 35,500 cubic feet per second.

Farmer said plans call for a gradual reduction down to 12,000 cubic feet per second by Dec. 11 because of the drought.

“We’re hoping Mother Nature brings some snow this winter,” she said, “but we’ve been told to expect low, stable conditions, that it’s probably going to remain dry.”

The cut in flow comes despite opposition from the governors of Missouri and Illinois and 77 members of Congress whose states sit along the Mississippi River.

The Mississippi is nearing historic lows between St. Louis and Cairo, Ill. Barges are already required to carry lighter loads and the middle of the river could be closed to barges if the level at St. Louis dips below minus 5 feet. It was at minus 0.45 feet Friday.

A zero river reading at St. Louis was established more than a century ago. It’s the point at which people at that time thought the river would never drop below.

The National Weather Service forecast for river levels extends only as far as Dec. 6. It calls for the Mississippi River to get to minus 3.7 feet at St. Louis by then. Businesses that ship on the river and their trade groups expect to get to minus 5 feet by around Dec. 10.

Barges carry 20 percent of the country’s coal and more than 60 percent of its grain exports. Other cargo, including petroleum products, lumber, sand, industrial chemicals and fertilizer, also gets shipped along the Mississippi.

Barge operators and those who ship on the Mississippi have warned that a shutdown would have disastrous economic consequences on those industries, with companies laying off workers if it lasts for any significant amount of time.

River shipping trade groups have even asked President Barack Obama to intervene.