The elements of maritime geography are loosely defined and their meanings have changed throughout history. The USA's 2010 Naval Operations Concept defines blue water as "the open ocean", green water as "coastal waters, ports and harbors", and brown water as "navigable rivers and their estuaries".[1] Robert Rubel of the US Naval War College includes bays in his definition of brown water,[2] and in the past US military commentators have extended brown water out to 100 nautical miles (190 km) from shore.[3]