ST. LOUIS — The Army Corps of Engineers has turned back requests by federal lawmakers and the barge industry to release more water from the Missouri River, believing the drought-starved Mississippi River it feeds still will remain open to shipping despite mounting concerns about water levels.

Army Assistant Secretary Jo-Ellen Darcy, in a Thursday letter obtained by The Associated Press, told lawmakers from Mississippi River states she doesn’t consider it necessary to boost Missouri River flows into the Mississippi — something the politicians urgently had sought.

Darcy, a top Army Corps official, noted this week’s revised National Weather Service forecast, which showed the Mississippi’s level wasn’t falling as rapidly as expected. She also said the corps is hastening its push to rid the river of rock pinnacles south of St. Louis that endanger barges when the water level is low.

Darcy also reinforced what the corps has been insisting for weeks: Reducing the Missouri’s flow is necessary because low levels in its upper basin could negatively affect recreation in the upper Missouri while impacting drinking water supplies, animal habitat and hydropower. Darcy added that the corps is legislatively bound to act in the best interest of the Missouri River, with what happens on the Mississippi incidental.

U.S. Sen. Claire McCaskill did not greet the letter warmly. The Missouri Democrat asserted Friday the corps would be to blame if shipping on the Mississippi — a corridor on which everything from grain to coal, chemicals and petroleum products is transported — gets slowed or shut down completely.

“Missouri businesses and jobs depend on our ability to continue commercial navigation along the Mississippi — and the dropping water level can’t be ignored,” McCaskill said. “The Army Corps is now saying that we can continue navigation without increased flows from the Missouri, and we should hold them accountable if that prediction doesn’t pan out.”

The corps last month began paring the outflow from an upper Missouri River dam in South Dakota with plans to gradually cut about two-thirds of the flow through next Tuesday. That action stoked concerns among Mississippi River barge operators, given that Missouri River water accounts for about 78 percent of the Mississippi at St. Louis.

Of chief concern is a pivotal 180-mile stretch of the Mississippi from St. Louis to the confluence with the Ohio River at Cairo, Ill., where heavy two-way traffic includes shipments going south to the Gulf of Mexico, as well as transports from the Illinois and Ohio rivers headed north to Chicago and Minneapolis. There, the Mississippi is 15 to 20 feet below normal due to months of drought, and rock pinnacles at two southern Illinois sites could make it difficult, if not impossible, for barges to pass if the river drops much lower.

Several Midwest lawmakers met privately with Darcy last week, asking her to analyze whether additional water from the Missouri can be released without sacrificing the corps’ objectives upriver from the South Dakota dam. Darcy’s letter was their answer — and to Democratic Sen. Dick Durbin of Illinois, an understandable one, in that the corps is bound legislatively to be the Missouri’s steward, independent of any consequences for the Mississippi.

“She believes releasing water could threaten the Missouri River navigation in 2013. That’s her technical conclusion, which is not good news for us downriver,” Durbin told The Associated Press. He said he would seek another meeting next week with Darcy, and would ask barge operators and other Mississippi-dependent industries to attend so Darcy can “spell out to them in as much detail as possible her vision of getting through this challenge without serious disruption to the economy.”

The Mississippi’s level at St. Louis was around 13 feet Friday — about four feet above the point at which the U.S. Coast Guard has said could necessitate further barge restrictions. Previous forecasts suggested the river could dip to the 9-foot level as early as Sunday, though the National Weather Service said this week it believes the river will stay above the 9-foot threshold until about Dec. 29.

As for the rock pinnacles, the corps originally planned to hire a contractor by early February to blow up the rocks, but at the request of lawmakers have hastened that effort, with Darcy saying it could begin this month.

All the while, the corps continues to look at options to ensure the Mississippi stays open.

Last month, the agency released water from an upper Mississippi River reservoir in Minnesota, adding a few inches of depth. Mike Petersen, a corps spokesman in St. Louis, said the agency also is considering reducing the water level at five lakes in Missouri and Illinois, allowing some of that water to flow into Mississippi tributaries. But Petersen said that option is complicated by the fact the drought has left four of those lakes too low for additional water removal.