Base level, in hydrology and geomorphology, is the limit below which a stream cannot erode. Upon entering a still body of water, a stream’s velocity is checked and thus it loses its eroding power; hence, the approximate level of the surface of the still water body is the stream’s base level. If a stream enters the sea, its base level is sea level; this is known as the ultimate base level. If a stream enters a lake, the lake level acts as a temporary base level for all parts of the stream above that elevation. All continental areas tend to be eroded down to the ultimate base level, or sea level, but uplifting of the Earth’s crust and variations in sea level prevent this from happening except in rare, small areas.