A tributary is a freshwater stream or river that flows into a larger stream, river, or other body of water. The larger, or parent, river is called the mainstem. Tributaries are important because they contribute to the mainstems larger overall watershed. Tributaries can be designated as "left bank" or "right bank" based on which side of the mainstem they feed into when an observer looks downstream. They can also be classified based on either their proximity to the headwaters of a river or the relative sizes of their flow. Tributaries should not be mistaken for "distributaries," which flow from instead of to the mainstem in what is known as "bifurcation".

Here are some additional key points about tributaries:

• Tributaries carry various sediment, chemicals, organic matter, and volumes of water that contribute to unique conditions that support various species.
• Cumulative pollution from a rivers tributaries contributes to unsafe habitats and drinking water.
• Tributaries are sometimes listed starting with those nearest to the source of the river and ending with those nearest to the mouth of the river.
• The Strahler stream order examines the arrangement of tributaries in a hierarchy of first, second, third, and higher orders, with the first-order tributary being typically the least in size.
• Not all tributaries flow throughout the year, as they may be affected by weather conditions.