A river is a natural flowing fluid course, flowing towards an ocean, sea, lake or another river. In some cases, a river flows into the ground and becomes dry at the end of its course without reaching another body of water.
A river begins at a source (or more often several sources) which is usually a ridgeline, drains all the streams in its drainage basin, follows a fluid course, and ends either at a mouth or mouths which could be a confluence, river delta, etc. The fluid in a river is usually confined to a channel, made up of a stream bed between banks.