Tributaries are streams or rivers that flow into a larger main river. This much larger river also is referred to as a main stem. A tributary also is called an affluent.
The place where tributaries join the main stem is called the confluence. Left-bank tributaries and right-bank tributaries refer to the side of a main river where the tributaries enter the river.
When many tributaries flow into a main river, it forms a very large river, such as the Mississippi River. An example of a tributary of the Mississippi River is the Missouri River, which also is its longest tributary. Some other tributaries of the Mississippi are the Ohio River and the Wisconsin River.