Floods close Mississippi River locks, halt barge shipments Thu May 30, 2013 6:15pm EDT (Reuters) - Heavy spring rains across the U.S. Midwest swelled the Mississippi River above flood stage at some locations and forced the closure of three river locks on the major shipping waterway, the U.S. Army Corps of Engineers said. At least four more locks were likely to be closed by early next week, disrupting the flow of grain barge shipments from the Midwest farm belt to export terminals at the Gulf of Mexico. This second wave of flood-related lock closures so far this spring comes just five months after near-record low water had threatened to close the river to commercial vessels. Locks 20 through 22, between Canton, Missouri, near the Iowa border and Saverton, Missouri, about 40 miles downriver, were shut down on Wednesday as floodwaters breached the lock gates, said Hilary Markin of the Army Corps. The latest river forecasts from the National Weather Service suggest lock 17 near New-Boston, Illinois, could be closed by Sunday and locks 16 and 18, at Muscatine, Iowa, and Gladstone, Illinois, may close by Monday, she added. Further downriver, lock 24 at Clarksville, Missouri, was likely to close on Friday, said Michael Petersen, spokesman for the Corps' St. Louis District. It was unclear how long the locks would remain closed as more rain was expected in the Midwest over the next several days, Corps said. Cash premiums for grain delivered by barge to Gulf Coast export terminals eased modestly on Thursday as pressure from a slumping pace of corn and soybean exports more than offset support from the temporary supply disruption, grain traders said. U.S. grain exporters rely on the Mississippi River and its tributaries to transport corn, soybeans, wheat and other farm products from production areas such as Iowa and Illinois to terminals near the Gulf of Mexico. Some 60 percent of all U.S. grain exports exit the country via the Gulf. Reporting by Karl Plume in Chicago; Editing by Kenneth Barry