A small river that flows into a large river is called a tributary. The tributary meets the parent river, named the mainstem, at a point called the confluence. Tributaries do not flow directly into oceans or seas.
Large rivers often are the result of a meeting of many tributaries. A tributary drains from a watershed, which is a geographic area that drains to one main waterway. Lakes, estuaries and streams are all waterways that drain out of a watershed. If tributaries carry the same name as the mainstem, they are called forks. Tributaries can be classified or ranked either by how close they are to the source of the main river or by their volume. An example of a notable tributary is the Missouri River, which is the most highly ranked tributary of the Mississippi River.