The overall water quality rating for the region is based upon the average High Gradient Macroinvertebrate Index (HGMI) Score for the 67 sites in the North and South Branch Raritan Watershed. The HGMI takes into account seven metrics that measure the presence, abundance, and pollution tolerance of the benthic macroinvertebrate community. Benthic macroinvertebrates are animals without backbones that live at the bottom of the stream, which you can see without the aid of a microscope, and serve as long-term indicators of stream health. HGMI is based on a scale from 0 to 100. HGMI Scores are rated as Excellent (63 – 100), Good (<63 to 42), Fair (<42 to 21), and Poor (<21).
The background of each major watershed is color coded by the average water quality ratings of the stream monitoring sites that fall within it. Each circle in the map represents a stream monitoring site. The individual sites are color coded based upon its individual water quality rating.