Streams supply some While storm winds are whipping up waves out on the open ocean, we're often pelted with rain and snow on land. Runoff heads downhill, picking up sediment on its way. Streams and rivers fill with runoff and carrying the dirty load downstream. Whenever the slows down it deposits some of its sediment load.
Most streams eventually reach the coast.
Here they dump their load of sediment into the surf zone where it is moved down the beach by longshore transport . View of Mobile Bay, Alabama area seen from Skylab 4. The sediment-laden Mobile River runs toward the sea. Where the brown fresh water reaches the surf zone sediment is transported by nearshore currents toward the west. Image from NASA's Earth Landscapes Cliffs are culprits too Along much of the west coast of North America, cliffs rise behind the shore.