River monitors were heavily armored, and normally mounted the largest guns of all riverine warships. The name originated from the US Navy's Brown Water Navy's USS Monitor, which made her first appearance in the American Civil War, and being distinguished by a single revolving turret. 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.