River Erosion (Stream Erosion)
As water enters the floor of a valley, it typically is dumped into a river or stream. Rivers and streams are moving bodies of draining water, that have a tremendous amount of force. Because of their strength, streams and rivers can cause a great amount of erosion.
Dirt and dust is carried away in the water of the river, leaving only pebbles and rocks. The rocks are constantly smacking into one another, as the force of the river moves them about. This causes them to be continually breaking into smaller and smaller pieces.
Rivers have been known to carve deep canyons in the bedrock in only a few hundred thousand years.