GRANITE CITY, Ill. (AP) — The busiest lock on the Mississippi River is back in business after days of emergency repairs caused a shutdown of that stretch of river near St. Louis, backing up hundreds of barges.

The U.S. Army Corps of Engineers reopened Lock 27 at Granite City at about 3:30 a.m. Thursday, five days after it was put out of action when crews noticed damage to what's called a "protection cell."

That's a rock-filled steel cylinder against which barges rub to help align them for proper entry into the lock. That buffer had split open, spilling enough of the rock into the river to obstruct passage.

By the time the lock finally was reopened, the Coast Guard says the traffic jam had grown to 63 vessels and 455 barges.