What is a river?
A river forms from water moving from a higher altitude to a lower altitude, all due to gravity. When rain falls on the land, it either seeps into the ground or becomes runoff, which flows downhill into rivers and lakes, on its journey towards the seas. In most landscapes the land is not perfectly flat—it slopes downhill in some direction. Flowing water finds its way downhill initially as small creeks. As small creeks flow downhill they merge to form larger streams and rivers. Rivers eventually end up flowing into the oceans. If people have built a dam to hinder a river's flow, the lake that forms is a reservoir.
The land surrounding and draining into a river is called its watershed or its drainage basin. Because all land lies within one watershed or another, all uses of land can have some impact on water quality.
Please visit the following links for more information on rivers and streams:
USGS Water Science School