Authorities will try to determine Saturday what caused two barges to break free on the Arkansas River in downtown Little Rock, prompting state police to shut down five bridges until inspectors determined that damage to two of them wasn't terribly serious. The runaway barges struck the Interstate 30 bridge that connects the downtowns of Little Rock and North Little Rock on Friday night and also hit the Clinton Pedestrian Bridge, part of the cities' riverfront trail system. "Out of an abundance of caution, the state police decided to close the highways," said Danny Straessle, a spokesman for the Arkansas Highway and Transportation Department. Two spans carrying city streets and another pedestrian also were closed after the towboat operator notified state police that he feared he had struck the bridges. The bridges were closed for about an hour, snarling traffic along the interstate and in the River Market entertainment district on the south bank of the river.