Rivers and streams are strong forces in shaping the landscape through which they flow. As the current moves against the channel and banks, the particles of sediment the river carries wear away the surface with a cutting action called erosion (ee-ROH-zuhn). All the world's rivers combined remove an average of 2 tons (1.8 metric tons) of rock and soil from every square mile (2.6 square kilometers) of land they cross each year. Some chunks can be huge. During a flood in 1923, a stream in the Wasatch Range of mountains in Idaho and Utah picked up boulders weighing as much as 90 tons (82 metric tons) and carried them more than 1 mile (1.6 kilometers) downstream.

The faster a river flows, the faster it wears the land away and the more sediment it bears. Some of the eroded chunks and particles may sink to the bottom. Others are...