Navy Patrol Boat The CUA VIET River is the supply line from the mouth of the CUA VIET (seen to the left of picture) to DONG HA for the 3rd DIV USMC. Navy patrol boats sweep it regularly to prevent NVA mines from disrupting river traffic. 1/61 conducts ambushes to the north of the river in an attempt to stop the mine layers as they move south. In fact the NVA have a special school for "Sappers", sort of like our Rangers, just across the DMZ north of CUA VIET and it is said that the graduation exercise is a patrol to mine the CUA VIET. Don't know if that is true but the mine laying parties do come south regularly.