The word stream describes a body of water moving through a channel until it reaches another stream, a lake, a sea, or the ocean. The movement of water, from higher-elevation or upstream regions to lower-elevation or downstream regions, is driven by gravity. Rivers are simply large streams. Streams and rivers can cover hundreds of kilometers and come in different shapes, from straight to curvy (or meandering). Very wide rivers that flow around numerous small or large islands can be described as braided or anastomosing.