More than a dozen industry groups tied to commerce on the Mississippi River are ramping up pressure on President Barack Obama and key federal agencies to add water to the Missouri River so commerce can continue on the middle and lower Mississippi River systems.

Work started last weekend to reduce outflow from dams on the upper Missouri in South Dakota is an “economic catastrophe” in the making if rocks aren’t removed from the Mississippi at Grand Tower, Ill., and Thebes, Ill., read a letter Tuesday from 18 trade groups representing the barge, grain, mining, steel and fertilizer industries, among others.

“We write to request your immediate assistance in averting an economic catastrophe in the heartland of the United States,” read part of the letter, addressed to the president, FEMA officials and the assistant secretary of the Army.

The Corps has reduced flow from Gavins Point Dam near Yankton, S.D., to protect the upper Missouri basin, a move opposed by political leaders in the middle Mississippi basin. The groups contend if an emergency isn’t declared and the Corps not directed to remove the rock pinnacles in Illinois and ease reductions of water to the Missouri, the middle and lower Mississippi River basin might drop to levels that could shutter barge traffic.

“Waterborne commerce on the middle Mississippi River between St. Louis and Cairo, Ill., will be severely impaired as early as mid-December unless the Administration takes emergency action to ensure that water levels do not fall below the level needed to support commercial navigation,” it reads.

Water levels on the lower Mississippi have fallen again in November after stages recovered slightly in October from drought conditions during most of the year in the Midwest. This morning’s stage in Vicksburg was 3.57 feet, down .29 of a foot. The river below St. Louis is predicted to fall farther in December, including a projected drop to below zero feet on the Vicksburg gauge. The record low stage locally is minus 7 feet, set in 1940.

If the federal government doesn’t step in, the impact could reach $7 billion in corn, grain, coal, petroleum, chemicals and other products delayed to market, the letter’s authors said.

“The time for action is now, because once the water levels on the Mississippi drop, this will be an even harder problem to solve,” said Tom Allegretti, president and CEO of American Waterways Operators, an Arlington, Va.-based group representing the nation’s tugboat, towboat and barge industry, in one of several statements from industry chiefs in a news release Tuesday.

The list of products the groups say are at risk in December and January includes 5 million barrels of crude oil that would have to be replaced by imports.

Navigation channels on the river must be kept 9 feet deep. Missouri Gov. Jay Nixon, Illinois Gov. Pat Quinn, Iowa Gov. Terry Branstad, 15 U.S. senators and 62 House members have written to the Obama administration to highlight the need for action to keep the river open to navigation.

“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.

Copies of the letter were sent to the president, FEMA administrator William Fugate and four of the agency’s regional administrators and assistant Secretary of the Army Jo-Ellen Darcy.

Co-authoring the letter were American Chemistry Council, American Farm Bureau Federation, American Fuel & Petrochemical Manufacturers, American Petroleum Institute

American Soybean Association, Institute of Scrap Recycling Industries, National Association of Manufacturers, National Corn Growers Association, National Grain and Feed Association National Mining Association, National Waterways Conference, North American Export Grain Association, Steel Manufacturers Association, American Waterways Operators, The Fertilizer Institute, The National Industrial Transportation League, U.S. Chamber of Commerce and Waterways Council Inc.