The National Park Service is beefing up its effort to stop the spread of zebra mussels in the St. Croix River. Since 1994, the federal agency has restricted access to boaters traveling north of Stillwater. Park Service officers monitored boat traffic from a houseboat near the Arcola sandbar 6 miles upstream. Now the Park Service is moving its checkpoint closer to Stillwater. Zebra Mussel Prevention Coordinator Byron Karns says the Park Service is trying to tighten up the checkpoint.