Explanation:
A stream or a small river that flows into a larger river is known as a tributary. A tributary does not flow into the sea or ocean but it joins a larger river or a lake. The larger river, in turn, flows to the sea or ocean. The tributary gets its water from rain, melting snow and other sources.

Importance of Tributaries:
Tributaries are an important part of a river system. They increase the volume of water in the main river and help in maintaining the flow of water. They also bring in nutrients and sediment that are important for the health of the river. Many species of fish and other aquatic animals depend on tributaries for spawning and feeding.

Examples of Tributaries:
Some examples of tributaries are:

- The Yamuna is a tributary of the Ganga river in India.
- The Missouri River is a tributary of the Mississippi River in the United States.
- The Rio Negro is a tributary of the Amazon River in Brazil.

Difference between Tributary and Delta:
A delta, on the other hand, is formed at the mouth of a river where it flows into the sea or ocean. It is a triangular-shaped area of land that is formed by the deposition of sediment carried by the river. Delta forms a fan-like shape and can be seen from a bird's eye view. It is not a tributary as it flows into the sea or ocean, unlike a tributary.

Conclusion:
Tributaries form an important part of a river system and are essential for the health of the river. They increase the volume of water in the main river and bring in nutrients and sediment. Understanding the difference between a tributary and a delta is important to understand the flow of water in a river system.