Already low water on the Mississippi River around St. Louis was expected by this weekend to approach levels too shallow for barge transportation, and a damaged lock slowed river traffic near Davenport, Iowa, river officials said Thursday. The St. Louis gauge for the Mississippi was at 3.0 feet on Wednesday and was forecast to hit 0.0 feet by Saturday. River traffic can continue operation through St. Louis until the gauge hits -3.5 feet and the river channel is 9 feet deep, the shallowest depth at which barge traffic can pass. "Any time it gets below five feet on the St. Louis gauge, everybody starts to monitor that. And when it gets below zero the river industry will start forming a low action water group and you'll see restrictions on tows and other things…