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Watershed Basics

Getting organized


Water quality

Water quality can be defined as the current status or condition of the water in a specific aquatic ecosystem.  It is much easier to describe what poor water quality is than to describe what conditions are considered good water quality.  Many of the lines between good and poor are stream-specific.  Each watershed has some natural buffering capacity.  This allows the water to adapt and compensate for normal changes in the environment such as leaching from the soil or the occasional heavy rain.


Water pollution

Pollution occurs when conditions exceed the watershed's ability to compensate for the changes.  Polluted water may be discolored , possess a coating on the bottom of the stream, or may show no visible sign at all of pollution. There are many different kinds of pollution.  The two major categories are Point Source and Non-Point Source pollution.

Point Source Pollution

Point source pollution comes from a defined, specific source such as a discharge pipe from a factory, municipal sewage treatment plant, or power generating station.  The state "Clean Streams Law" as well as the federal Clean Water Act have made great strides towards identifying, controlling and cleaning up point source pollution.  This category of pollution still has an effect on today's watersheds, but not to the great extent as prior to legislation.

Non-Point Source Pollution

Now that much of the point-source pollution is being controlled, problems are arising that were previously overshadowed.  Non-point source pollution (NPS) is being recognized as a major factor in the deterioration of today's watersheds. NPS pollution is covered by Section 319 of the Federal Clean Water Act.  A significant federal and state bureaucracies have been built up around NPS and Section 319.  It's common to refer to programs or funding simply by using "319" as the qualifier.

The most common types of non-point source pollution are: AMD, agriculture, erosion and sedimentation, and acid rain.  AMD is by far the worst problem facing area watersheds.  For more information on what AMD is see What is AMD?

Agriculture is a very serious source of NPS because there are so many kinds of pollution generated.  The two most likely pollutants from agriculture are nutrients and sediments.  Many of the pesticides and fertilizers used today have a tendency to be washed off of plants and filter into waterways through runoff, increasing nutrient loads.  Nutrient and sediment levels increase when unprotected streams run through livestock pastures.  Cattle are often the worst of all livestock offenders.  They trample the streams sides, increasing erosion and sedimentation.  Evidence of nutrient pollution is often seen as algal blooms.  These are large pockets of algae that make the water appear almost a soupy green.    Blooms have devastating consequences for aquatic organisms.  As the algae flourishes on the increase of nutrients (especially nitrogen), the amount of oxygen in the water rapidly decreases to nearly 0.  This leaves the water uninhabitable and results in widespread kills of fish and other organisms that are in the bloom path.

Erosion and sedimentation are also very common pollutants.  Often excess amounts of solids enter waterways because of run-off.  Constructions sites, fallow fields, and other areas of unprotected soil are extremely prone to large amounts of erosion.  Poor forest management practices, such as clear cutting a hillside can also result in increased erosion.  One methods of decreasing erosion and sedimentation is with the protection and establishment of riparian buffers.

Acid rain or acid precipitation, is becoming a common pollution source.  Car exhaust as well as other discharges spout compounds into the air.  As clouds form and water vapor mixes with the gases, acids are formed.  Sulfuric and nitric acids are the two most commonly found.  When the clouds release the water as precipitation, these acids are carried down to earth and drain into unsuspecting waterways.  There are times when a stream may appear to be suffering from AMD by the pH level, however no discharge can be found.  Acid rain can have the same profound lowering effect on pH as AMD.  It may also carry minerals and metals into the stream that the acidic pH leached out of surrounding rocks.

Storm water runoff is also a problem.  Though it may not contain many pollutants, the unrestricted dumping of a large volume of water is detrimental in itself.  The streams are accustomed to gradual flow into a waterway, not the forceful rush often associated with storm drains.  The extra flow raises levels above the normal often causing flooding and erosion.  The increased flow of water also impacts aquatic habitat, changing the character of the stream from a quiet pool to a rushing, tumbling surge.  Many organisms will not inhabit such fast-moving water.


A Watershed Primer for Pennsylvania, produced by the PA Environmental Council, has many related essays detailing various kinds of pollution and treatment methods.

Water Quality WebQuest provides a good general resources on water quality and nutrient cycling.

The National Water-Quality Assessment Program provides details on various pollution types and water assessment methods.

The Water Quality Information Center provides a library of water quality information available for searching.

PA's Fish and Boat Commission provides a general over view on the Basics of Water Pollution in Pennsylvania.

CreekConnections, a project of Allegheny College, provides a variety of modules for learning about PA's streams, rivers, and waters.