A tributary  or affluent  is a stream or river that flows into a larger stream or main stem (or parent) river or a lake.  A tributary does not flow directly into a sea or ocean.  Tributaries and the main stem river drain the surrounding drainage basin of its surface water and groundwater, leading the water out into an ocean.
"Right tributary" and "left tributary" (or "right-bank tributary" and "left-bank tributary") are terms stating the orientation of the tributary relative to the flow of the main stem river. These terms are defined from the perspective of looking downstream (in the direction the water current of the main stem is going). 
In the United States, where tributaries sometimes have the same name as the river into which they feed, they are called forks. These are typically designated by compass direction. For example, the American River receives flow from its North, Middle, and South forks. The Chicago River's North Branch has the East, West, and Middle Fork; the South Branch has its South Fork, and used to have a West Fork as well (now filled in).
Forks are sometimes designated as right or left. Here, the "handedness" is from the point of view of an observer facing upstream. For instance, Steer Creek has a left tributary which is called Right Fork Steer Creek.
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. For example, a second-order tributary would be the result of two or more first-order tributaries combining to form the second-order tributary. 
The Liao River is a much simpler example of a river basin with tributaries. The main tributaries noted on this map are the Hun River, Taizi River, Dongliao River, Xinkai River, Xiliao River, Xar Moron River and the Laoha River. The Xiliao River has the tributaries on the map the Xar Moron and Laoha Rivers.
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- "opposite to a tributary". PhysicalGeography.net, Michael Pidwirny & Scott Jones, 2009. Viewed 17 September 2012.
- Bisson, Peter and Wondzell, Steven. “Olympic Experimental State Forest Synthesis of Riparian Research and Monitoring”, United States Forest Service, p. 15 (December 1, 2009).