What is a Watershed?
A watershed is an area of land that drains water into a stream or other body
of water. Watersheds exist at many different scales and are named for the
draining water feature. The New River Watershed, for example, includes all of
the land surfaces and streams over which water eventually flows to the New
Watershed conditions can directly affect the quantity and quality of source
water supplies. Water travels over the land’s surface – farm fields, forests,
lawns and city streets – on its course to a waterway. The water is impacted by
what happens in the watershed.
The climate and amount of precipitation determine how much water is available
and the lay of the land controls how water and other materials move through the
watershed. The type of land use and development often relates to quality because
certain contaminants can be associated with specific land uses. Understanding
local water conditions is an important component of source water assessment and
For more information about watersheds and watershed management, visit the
USEPA’s Watershed Academy at:
Potential Sources of Contamination
Various potential sources of contamination or pollution can exist within
watersheds. These can be from either natural sources such as minerals that leach
out of soil and rock, or man-made sources such as manufactured chemicals or
waste materials. Contaminants can enter water supplies by direct runoff,
contributing flow from smaller streams, and/or infiltration into the ground.
Contaminant sources are often grouped into one of two broad categories: point
sources and non-point sources.
Point source pollution occurs at a
specific identifiable location, such as discharge from a pipe or a leaking
aboveground or underground storage tank.
Non-point source pollution
(NPS) occurs across a broader area that cannot be tied to a specific
point. Examples of non-point source pollution include excess fertilizers and
pesticides from agricultural and residential areas, waste and toxic chemicals
from urban stormwater runoff, and sediment from erosion.
Each watershed is unique and can contain different types of contaminant
sources depending on land use. Source water assessments evaluate local
conditions and support development of strategies to manage risk to sources of
supply for drinking water.
For more information about pollutants and management strategies, visit the
USEPA’s Water Pollution Prevention & Control site at: