Since 1969, the U.S. Geological Survey and the Chester County Water Resources Authority have conducted a cooperative program to evaluate stream ecology and water-quality conditions using benthic macroinvertebrates and stream-water chemistry. The Stream Conditions of Chester County Program has sampled streams every fall for over 30 years. The initial goals of the program were to evaluate stream-water quality and to further the understanding of changes in the stream ecosystem in response to urbanization. The current goals of the program are to use the data to monitor current conditions and determine trends. Data from the program have been used to help support Chester County Landscapes by providing information on biological diversity and water-quality conditions. Without monitoring it is impossible to determine if changes in land use or environmental policies and regulation are having a positive affect on water quality.

Benthic macroinvertebrates are macroscopic animals that inhabit the bottoms of aquatic habitats. Freshwater forms include aquatic insects and other invertebrates including clams, crustaceans, snails, and worms. Factors such as streamflow, food availability, habitat, temperature, and water quality determine the makeup of the macroinvertebrate community. By sampling in similar habitats with similar physical conditions, water quality becomes the determining factor controlling community structure. Changes in abundance, diversity, species richness, and presence or absence of pollution tolerant or intolerant species can be measured and related to water quality. Trends in water quality can be determined by sampling at a single location over several years and observing changes to the community structure.

Benthic macroinvertebrates are well suited as water-quality indicators because of their biology and availability. They are present in most aquatic systems and are relatively easy to collect. They are indicators of overall water quality and can be used to identify specific types and sources of degraded water quality. Benthic macroinvertebrates have limited mobility and cannot avoid poor water-quality conditions. They are sensitive to a wide range of environmental impacts including chemical and physical impairments.

Biological samples collected for the Stream Conditions of Chester County Biological Monitoring Network consist of benthic macroinvertebrates collected from a riffle area of the stream. A riffle habitat is used because macroinvertebrate diversity and abundance is usually highest there.

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