Abstract This study analyzed change of a coastal wetland using remote sensing, image processing, and GIS techniques. Aerial photographs from 1928, 1945, 1975, and 1994 were acquired for San Dieguito Lagoon, in San Diego County, California, USA. The photographs were scanned and the landscape classified as wetland or non-wetland. A GIS model quantified and indicated trends in wetland change. Results show that, in 1928, 50% (366 ha) of the study area was comprised of wetlands. By 1994, the total wetland area decreased to 15% (109 ha) of the study site area. The annualized loss reveals a continual decrease in the rate of loss over all three time periods, which corresponds to the national decline in the rate of wetland loss. The greatest loss rate occurred from 1928 to 1945 (3% per year), and the lowest rate of loss occurred from 1975 to 1994 (0.3% per year).