The outlet of a watershed, or drainage system, is the mouth of the main stream or river. Its mouth may be where it flows into another river or stream, or the place where it empties into a lake, estuary, or ocean. Like streams and rivers, watersheds vary in size. A watershed can be very small, perhaps only a few square kilometers such as one that drains a small stream in your neighborhood. Or it can be very large, like the 668,220 square kilometer Columbia River watershed, or the even larger Mississippi River watershed.
Tributaries are smaller feeder streams of rivers which empty into a larger mainstem river .
Thus large watersheds draining into rivers such as the Columbia or Mississippi may be made up of many smaller sub-watersheds. The smaller sub-watersheds may, in turn, be made up of even smaller watersheds that are fed by smaller tributaries. Any given watershed may be part of a larger watershed and itself be made up of smaller watersheds. Topography , the three-dimensional shape of the land surface, determines the boundaries of all watersheds. The topographic boundaries of a watershed are called divides .