A river is a natural flowing watercourse, usually a freshwater stream, flowing on the earth's land surface or inside caves towards another waterbody at a lower elevation, such as an ocean, sea, bay, lake, wetland, or another river. Rivers can be of different sizes, and local terms can cause the actual definition of a river to be vague. A river begins at a source, drains all the streams in its drainage basin, follows a watercourse, and ends either at a mouth or mouths, which could be a confluence, river delta, etc. The water in a river is usually confined to a channel, made up of a stream bed between banks. Rivers are important for many reasons, including their role in the water cycle and shaping the Earth through erosion and creating geological formations such as canyons