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The study attempts to provide additional understanding of the long-term evolution of water quality in the Pearl River estuary and Hong Kong coastal areas by monitoring the ocean colour from remote sensing images. The extensively inter-connected river network of Pearl River estuary has a large catchment area and its environmental deterioration due to rapid economic growth is significant. The data received by Landsat satellites, over a time period of more than 20 years, were used to evaluate the evolution of water clarity. The study shows the historical varying of sea colour in these areas and image processing methods for extracting the ocean colour features of Landsat images are suggested. The algorithm of 'Gradient Transition' was used to calibrate the result images and the comparing method was used to process and analyse remote sensing images of Case 2 water. [ABSTRACT FROM AUTHOR]