Macroinvertebrates are organisms that do not have a backbone and are visible without a microscope. In most streams, the energy available to organisms is stored in plants and made available to animal life either in the form of leaves and algae that are eaten by macroinvertebrates. In turn, the macroinvertebrates are a source of energy (food) for larger animals such as fish, which are a source of energy (food) for birds, raccoons, and humans.

Macroinvertebrate sampling is coupled with chemical water quality sampling. The reason for this is quite simple.  The process of identifying stream pollution with water analysis alone is time intensive and can only provide limited information from the time of sampling. Even the presence of fish may not provide information about a pollution problem because fish can move away to avoid polluted water and then return when conditions improve. However, most stream-bottom macroinvertebrates cannot move to avoid pollution. Therefore, a macroinvertebrate sample can serve as an indicator of water quality by providing more information about pollution or the overall water quality that is not present at the time the sample is collected.

Some stream-bottom macroinvertebrates cannot survive in polluted water while others can survive or even thrive in polluted water. For example, species commonly found in colder waters (see temperature) that need high levels of dissolved oxygen include pike, trout, bass, salmon, mayfly larvae, stonefly larvae, and caddisfly larvae are indicators of a healthy stream. Species commonly found in warm waters such as sludge worms, black fly larvae, and leeches are more tolerant of low dissolved oxygen levels, indicators of an unhealthy stream. Additionally, useful stream-bottom macroinvertebrate data are easy to collect without expensive equipment.

Finally, a healthy ecosystem supports diversity of organisms, so in a healthy stream, the stream-bottom community will include a variety of ALL of the pollution-sensitive macroinvertebrates. Conversely, an unhealthy stream will support only a few types of non-sensitive macroinvertebrates.

A stream with excellent water quality should support organisms
from ALL three (3) pollution tolerance groups.