The health of fish and macroinvertebrate (i.e., animals without backbones that are larger than the size of a pencil dot) communities is a strong indication of overall stream health.

Since the mid–1990’s many natural resources professionals working in the Anacostia watershed have monitored these communities using an Index of Biotic Integrity or IBI approach. The IBI compares the fish and macroinvertebrate communities of urban streams with those of healthy reference streams, incorporating geographical, ecosystem, community, and population, as well as distribution and abundance variables that account for differences in waterbody size, type, and region of occurrence.