RedactedNarrative: The U.S. Army Corps of Engineers (USACE) was finishing dredging operations near the northwest corner of Crater Island on the Mississippi River Pool 8 in Vernon Co., WI.  The USACE crew secured the upstream end of the empty dredge tube with spuds driven into the riverbed and took a lunch break in the galley of the USACE barge.  The spuds came loose and the dredge tube floated into the navigation channel.  Boat A was heading upstream on the Mississippi River in the center right of the navigational channel between mile markers 690 and 691.  Boat A was traveling approximately 10 miles per hour.  Boat A traveled between the USACE tug boat and a barge, and went over the floating dredge tube.  The dredge tube struck Boat A in the center of the bottom of the boat and in the lower unit.  Contact between the floating dredge tube and Boat A resulted in the lower unit of Boat A being broken off the transom.