Rivers, streams and creeks are a surface water system with a network of channels that collect and move runoff. Runoff is excess water that is not adsorbed by the surrounding area. Runoff can be created by rainfall, melting snow, or groundwater discharge. The shape of the river system will depend on the topography, the type of rock or soil it is traveling through, and the style of its biological diversity. For instance, pine trees require a lot of water to grow. Runoff in a pine tree forest would be less than if the area was populated by oak trees. Oak trees require less water than pine, so more water would be available as runoff.