Rivers and streams are part of the Earth's hydrologic cycle. The hydrologic cycle describes the manner in which molecules of water evaporate, condense and form clouds, and return to the Earth as precipitation (rain, sleet, or snow). Rivers pass through several stages of development.

Formation

Rivers and streams owe their existence to precipitation, lakes, and groundwater, combined with gravity and a sloping terrain.

When rain falls on the land, often the soil cannot absorb it all. Much runs off and with the aid of gravity travels downhill, creating rills (tiny gullies). Many of these rills may meet at some point and their waters run together to form bigger gullies until eventually all this water reaches a valley or gouges out its own large channel. When enough water is available to maintain a steady ongoing flow, a stream or river is...