Flowing water, in streams and rivers or across the land in sheets, is the dominant erosional process in shaping Earth's landscape. Streams and rivers are not merely systems for moving surface water to the world's oceans and seas. They are also systems for moving weathered rocks and other sediment to those large bodies of water. In fact, it is estimated that streams and rivers move about 1.65 billion tons (1.5 billion metric tons) of sediment from land to the oceans each year. By shifting such great masses of sediment, streams and rivers become sculptors of the land.
Streams and rivers erode, transport sediment, change course, and flood their banks in natural and recurring patterns. It is true that most of the erosional work done by surface water is not done by streams or rivers but instead by falling raindrops and by the resulting unorganized runoff down slopes. Yet streams and rivers are able to create both erosional landforms (their own channels, canyons, and valleys) and depositional landforms (floodplains, alluvial fans, and deltas) as they flow over Earth's surface.