Rivers in the Solar System; Water is not the only fluid flow on planetary bodies.

Water, either in liquid or ice form, is the most common fluid that forms channels and valleys on Earth. Lava flows also make channels. The responsible fluid is generally clear on Earth because the process of formation tends to be observable. Channels and valley landforms are widespread on surfaces of planetary bodies, but their formation processes are not normally observable in action. Therefore, planetary geologists rely on geomorphology and knowledge of the environment to infer the nature of the fluid that produced these features. A wide range of fluid flows, including water, lava of various compositions, carbon dioxide, and hydrocarbons, have been proposed for the genesis of channels and valleys on the Moon, Venus, Mars, Io, and Titan.