The Georgia Department of Natural Resources on Monday was investigating a weekend boating accident on the Gwinnett County side of Lake Lanier involving a teenage girl who suffered a 16-inch cut to her left thigh. The 16-year-old, whose name was not available, was climbing into a boat from the stern Sunday when she fell and suffered the injury, which was caused by a propeller, DNR spokesman David G. Allen said. “The vessel was running but was in neutral,” Allen said.