StreamWatch BATS monitors the populations of organisms called macroinvertebrates. Water quality ratings are based on the abundance of various species and their sensitivity to pollutants. If the stream is healthy, a diverse macroinvertebrate population can be found in a relatively small sample of water. A reduction in their numbers may indicate a high level of pollution.
Sampling is conducted during a six-week sampling season in the spring of each year. Beginning in 2023, all the samples collected by volunteers are sent to a certified lab for identification to the genus level.