The locations of waterline in remotely sensed images are mainly affected by high concentration suspended sediments and surface remnant water. A decision tree model considering the water depth was applied in this paper to detecting waterline. Furthermore, waterline was also traced from the reference digital terrain model (DTM) and the associated tidal elevation. The two approaches were both used to delineate the waterline in the Yangtze Estuary, and the experimental results indicate that they are fairly effective in waterline extraction.