Temperature changes cause pieces of rock to flake away from the surface. Also, the acid in rainwater dissolves rocks containing calcium carbonate. These processes are called weathering.
Water erosion happens when water moves the pieces of rock or soil downhill. Waves also carry away small pieces of material. A wave can wash up onto the surface of rock or soil and then carry away pieces of material as it flows back into the ocean or lake.
The size of earth materials that can be moved by water depends on how fast the water is moving. A fast-flowing stream can carry large rocks while a slow-moving stream might only be able to carry very small things like clay. Canyons are among the most obvious features made by erosion. Where a river meets the sea, it drops the solids, sometimes making a river delta.
Large tropical rivers like the Paraná, Indus, Brahmaputra, Ganges, Zambezi, Mississippi and the Amazon carry huge amounts of sediment down to the sea. The Nile, perhaps the world's longest river, carries much less sediment than the others because, part of the way, it runs through less fertile regions than the other great rivers.

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