Macro-invertebrates are aquatic animals without backbones that are visible to the naked eye, including the larval and nymph stages of many insects. Because different species have well-known tolerance levels to pollution, temperature, and oxygen, their presence, absence, and diversity provide a reliable, science-based measure of stream health.

Following standardized MICorps procedures, trained HLPOA Environmental Committee volunteers conduct biannual macro-invertebrate surveys at established sites. A consistent scoring system allows results to be compared year-to-year to track trends in water quality and ecosystem health.