ST. LOUIS - U.S. Sen. Roy Blunt of Missouri said Tuesday he is weighing whether to ask President Barack Obama for an emergency declaration in an effort to keep barges moving on the drought-riddled Mississippi River.

The Army Corps of Engineers last week began reducing the outflow from an upper Missouri River dam in South Dakota, a move the corps said was necessary to ease drought conditions north of the dam.

That means less water coming down the Missouri and into the Mississippi River. The already-low Mississippi River level is dropping as a result and could get so low that barge traffic will be severely restricted or even halted by mid-December between St. Louis and Cairo, Ill.

Nearly 80 members of Congress, the governors of Missouri, Illinois and Iowa and many waterway operators have written to the Army, citing concerns that shutdown of the Mississippi could create an economic crisis not only for the barge industry and those who ship on the river, but for consumers.

The flow from Gavins Point Dam near Yankton, S.D., was at 37,500 cubic feet per second on Friday when the flow reduction began. It is being gradually reduced to 12,000 cubic feet per second by Dec. 11.

Corps officials in Omaha, Neb., have said the reduction is necessary because low water in the upper Missouri River basin is affecting recreation, exposing Native American artifacts that are normally covered by water, and may eventually impact hydropower. They say they are bound by the Missouri River Master Manual to act in the best interest of the Missouri River, and what happens on the Mississippi is incidental.

Blunt, speaking during a telephone news conference, said the move was another example of "friction" between the upper basin of the Missouri River and the lower Missouri and Mississippi rivers.

"So I think the next step there is to evaluate that if the corps is not responsive whether we should ask the president for a declaration of economic emergency," the Republican senator said.

River interests already are seeking such a declaration. Three trade groups for the shipping and barge industry along with 15 other national organizations submitted a letter Tuesday to Obama and to the Federal Emergency Management Agency. It requested a presidential emergency declaration that would require the corps to immediately remove rock formations that threaten barge traffic near Thebes, Ill., and Grand Tower, Ill., and require the corps to maintain enough flow down the Missouri River to maintain a navigable channel on the Mississippi.

"This is an economic disaster in the making and the administration needs to act now to stop it," said Mike Toohey, president and CEO of Waterways Council Inc.

The corps plans to remove the rock formations but has said the bidding process will take several weeks to complete. The work is scheduled to begin in early February.

The letter warns that closure of the river in December and January would place at risk $7 billion in goods that include corn, grain, coal, petroleum and chemicals that rely on the Mississippi for shipment.