The U.S. Geological Survey is using GIS to assess the trophic state (health) of more than 50 Massachusetts lakes and ponds, which were selected to represent a wide range of water quality conditions. The procedure correlates GIS-based gridded field maps of the present distribution of submerged, emergent, and floating algae and plants (macrophytes) with Thematic Mapper (TM) satellite data. The correlations of the TM data with the field data on macrophyte distribution will be used to infer the trophic states of more than 3,000 lakes and ponds in the State. In addition, archived TM data and historical near-infrared photographs will be used to identify trends in macrophyte distribution.