DEQ's Freshwater Biological Monitoring Program uses the benthic macroinvertebrate community to assess the ecological health of freshwater streams and rivers. Benthic macroinvertebrates are invertebrate organisms, such as insects, crustaceans, snails or worms that live on the bottom of streams and rivers that are large enough to be seen with the naked eye. As many of the organisms that make up these biological communities are extremely sensitive to pollutants, they often respond to changes in water quality caused by the introduction of various contaminants from point or non-point source pollution. In essence, benthic macroinvertebrates are considered to be virtual "living recorders" of water quality conditions over time.

Analysis of these organisms provides a measure of the overall water quality of a particular water body segment. The assessment is made by comparing the community of benthic macroinvertebrates collected at a "reference" stream, where there are no significant disturbances, to the community of benthic macroinvertebrates collected at the stream under consideration.