The U.S. Army Corps of Engineers rejected lawmakers' request to increase the flow from a major tributary of the Mississippi River, where declining water levels threaten the shipment of about $7 billion in cargo.

The Corps lacks authority to adjust the Missouri River flow to benefit Mississippi navigation, Jo-Ellen Darcy, the Army's assistant secretary for civil works, said in a letter to Senator Richard Durbin, an Illinois Democrat, released Friday.

"The Corps has identified the potential for significant negative effects on the Missouri River system," including a depletion of drinking water supplies, loss of marine-wildlife habitat and higher bills for users of hydro power, Darcy said.

Barges carrying grain, soybeans, coal, oil and other commodities on the Mississippi River have started to reduce their loads to navigate waters shrunk by the worst drought in 50 years. By the year's end, rock structures in the river near southern Illinois threaten to curtail traffic as the river recedes, according to a forecast Wednesday from the National Weather Service.

Water levels along the 2,300-mile Mississippi will approach record lows just after the first of the year in the absence of rain. Companies that rely on the river to transport goods, including Archer-Daniels-Midland Co. and American Electric Power Co., may see a decline in their cargo shipments.

Darcy on Nov. 29 agreed to expedite a Corps review of the impact of an increase in the water flow from the Missouri River, which joins the Mississippi near St. Louis. The Corps also plans to begin demolishing submerged limestone "pinnacles" near the Illinois town of Thebes "as early as possible in December," according to the letter. The Corps planned to begin the work in February.

Barge traffic starts being impeded when the river channel falls below a depth of about 9 feet, according to the Corps. The water level at St. Louis won't reach that point until Dec. 26, according to the National Weather Service.

The rain now forecast, expedited removal of the rocks and limited flows authorized from upstream "are expected to be sufficient to sustain navigation" on the middle Mississippi River, without additional releases from the Missouri, Darcy said in her letter to Durbin.

Shippers are urging immediate action because any water released from the Missouri River takes about two weeks to raise the level of the Mississippi, according to the Waterways Council, based in Arlington, Va. The industry group estimates about $7 billion in commodities travel along the river each December and January.