Water erosion occurs on stream and river banks, seashores and seaside cliffs. Muddy water is a sign of erosion. The brown hue results from the suspension of rock and soil particles in the water.
Moving water, one of the primary agents of erosion, carries soil particles and rock fragments to other places. When moving water slows down, the transported material, which is called sediment, becomes deposited in a new area. The sediment starts to accumulate and creates fertile land. Delta sediment that is carried from river beds and banks forms river deltas.
Rushing rivers and streams slowly erode their banks and create continuously expanding valleys. An example of an eroded river is the Colorado River, which bored deep into the land and gradually formed the Grand Canyon over 5 million years.
Water erosion reshapes coastlines through waves that incessantly smash against the shores, pounding rocks into pebbles and turning pebbles into sand. Water also tends to reduce the sand in beaches, thus moving the coastline farther inland.
Strong ocean waves wear away seaside cliffs and sometimes create holes that become caves. Water forms an arch when it enters the back of the cave. The restless crashing of the waves sometimes breaks the top of the arch. The remaining rock columns are called sea stacks.