River monitors are military craft designed to patrol rivers. They are normally the largest of all riverine warships in river flotillas, and mount the heaviest weapons. The name originated from the US Navy's , which made her first appearance in the American Civil War, and being distinguished by the use of revolving gun turrets, which were particularly useful in rivers, whose narrow channels could severely limit the directions vessels could face. River monitors were used on inland waterways such as rivers, estuaries, deltas and lakes. Usually they had a shallow draft which was necessary for them to be able to operate in enclosed waters; but their displacement, size and draft varied depending on where they were used. Most river monitors were lightly armored although this varied, with some carrying more armor. Exceptional examples, however, most notably the Royal Navy's s, which could operate in coastal or certain riparian/estuarine situations, bore extra-thick armor plating and heavy shore-bombardment guns, up to a massive 18 inches (457 mm) in size. Typically, however, river monitors displayed a mixture of gun sizes from 3-inch (75 mm) to 6-inch (152 mm), plus machine guns. This type of vessel overlaps with the river gunboat that would be armed with relatively small caliber cannons, or a mix of cannons and machine guns. United States River monitors were used during the American Civil War, playing an important role in the Mississippi River Campaigns. They also played a role in the Battle of Mobile Bay. The American Civil War river monitors were very large, weighing up to 1,300 tons. On 18 December 1965, the US Navy, for the second time in one hundred years, authorized the reactivation of a brown-water navy for riparian operations in South Vietnam. In July 1966, Secretary of Defense Robert S. McNamara authorized the formation of a Mobile Riverine Force (MRF); a force that would bring back the river monitor. The US Navy operated its Monitors as part of their River Assault Flotilla One, which initially consisted of four River Assault Divisions (RAD); with RAD 91 containing three monitors, RAD 92 having two monitors, RAD 111 having three monitors, and RAD 112 operating two monitors. The Vietnam monitors were divided into two programs; program 4 would consist of the 40 mm gun monitors, while the later program 5 would entail the eight Monitor (H) Howitzer versions, and the six Monitor (F) Flamethrower models. All of the