Rills, gullies, streams, rivers and tributaries are all caused by

All these bodies of water are formed when water flows from the top, such as from a hill, mountain, or down a valley, etc. In these high areas, melting snow causes tiny streams of water to flow down. They are also formed when small bodies of water combine to form larger bodies. For instance, A stream is a channel along which water is continually flowing down a slope. Unlike gullies, streams rarely dry up. Small streams are also called creeks or brooks. As streams flow together, they form larger and larger bodies of flowing water. A large stream is often called a river.