Watersheds can be used to define large drainage areas or small ones. This is a small watershed in comparison to that of all the tributaries of the Mississippi River , which eventually drains into the Gulf of Mexico. Image courtesy of Dr. Trevor Harris. Streams within a watershed are ranked by relative size and structure. The rank of a stream is referred to as stream order.
Stream order is a very important part of the watershed approach to examining water quality.
It enables one to examine the quality of streamwater at a particular site with that of a stream of similar size and structure at another point in the watershed. The image below is a computer-generated topographic map of Wheeling Creek Watershed. Computer-generated topographic map of Wheeling Creek Watershed. Image courtesy of Dr. Trevor Harris. One model for changes in biological communities that might take place as water travels from small headwater streams to larger rivers is referred to as the River Continuum Concept .