The 120 foot long towboat Yvonne Conway was proceeding downstream on the Ohio River with 12 empty barges when one of the barges struck the pier of the northbound bridge located between Henderson, Kentucky and Evansville, Indiana.   The impact caused 7 of the barges to break free, but later recovered.   Traffic on the bridge was stopped for several hours until an inspection team inspected the bridge for damage.  An investigation into the incident is being conducted by the Coast Guard.