A watershed map – also called a drainage area, catchment, or basin – is an area of land that drains surface and subsurface runoff to a common point, usually a stream, river or lake.  Every piece of land is part of a watershed, because water always moves off of land to a lower point where it collects in a water body.  Watersheds may range in size from less than an acre to thousands of square miles in area.

There are a number of features that are characteristic to all watershed.  Watershed divides are the boundaries that define a watershed and separate adjacent watersheds.  The highest point or points in the watershed are called the headwaters and are the points at which the ditch, stream or river starts.  The lowest point in the watershed is called the base level, which is usually the mouth of the river or steam.  The difference in elevation between a stream’s headwater and its base level create the stream’s gradient.