By Karl Plume
June 3 (Reuters) - The Mississippi River was effectively closed to barge traffic from northern Iowa to St. Louis as flooding shuttered several locks on the major shipping waterway and stranded at least 70 barges, the U.S. Coast Guard and Army Corps of Engineers said.
The Coast Guard officially closed the busy Port of St. Louis from mile markers 179 to 184 due to high water and warned mariners about increased debris in the river.
The second wave of spring flooding to hit the Midwest comes just five months after near record low water on the Mississippi River threatened to halt barge shipping earlier this year.
The latest lock closures, some of which could remain in place until next week, choked off the flow of grain barges from Midwest farm areas to export terminals on the Gulf Coast, where about 60 percent of all U.S. grain exports exit the country.
Numerous grain elevators along the rivers were also unable to load barges as the vessels could not fit below loading spouts due to the river's higher level.
Seven locks, from Lock 17 at New Boston, Illinois, to Lock 25 at Winfield, Missouri, were currently closed and an eighth, Lock 16 at Muscatine, Iowa, was likely to close by Tuesday, the Army Corps said.
The latest river forecast suggests the locks could reopen by the middle of next week, although the timeline may change as more rain was expected in the region.
"These estimates are based on the latest river forecast and the amount of time it takes them to clean up any of the debris, especially at some of the lower locks," said Hilary Markin of the Army Corps Rock Island District.
The Illinois River was above flood stage at many locations but its locks remained open and crests were expected around midweek.
Eight vessels towing a total of 70 barges were waiting at locks between Mississippi River mile markers 273 and 325 as of midday Monday, the Coast Guard said.
Still, cash premiums for grain delivered by barge to Gulf Coast export terminals were largely unchanged as pressure from a slumping pace of corn and soybean exports offset support from the temporary supply disruption, grain traders said. (Reporting by Karl Plume in Chicago; Editing by Nick Zieminski)