<b>River</b> long watercourse that flows down a slope along a bed between banks. It originates from a ‘source’ and culminates in a sea or lake at its ‘mouth’. Along its length, it may be joined by smaller rivers called ‘tributaries’. A river and its tributaries form a ‘river system’. Land surfaces are never perfectly flat, and as a result, the runoff water after precipitation tends to flow downward by the shortest and steepest course in depressions formed by the intersection of slopes. Runoff water of sufficient volume and velocity joins to form a stream that, by the erosion of underlying earth and rock, becomes deep enough to be fed groundwater or when it has as its source an ultimate water reservior. For example, the Ganges flowing from the Gangotri Glacier and the Brahmaputra from the Manas Sarovar.