Complex sentence: The Davis lock is used rather infrequently to lock light freighters, tour boats, and small craft when traffic warrants or the MacArthur or Poe Locks are out of service for maintenance.
Split into simpler sentences: The Davis lock is used rather infrequently to lock light freighters, tour boats, and small craft. When traffic warrants or the MacArthur or Poe Locks are out of service for maintenance.