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[Stockbild] Estuarine water. Aerial photograph of the Alligator River Delta in Kakadu National Park, Northern Australia. Mangroves are seen lining the shore (bottom left to upper right) and along the banks of the rivers. The water at upper right drains from a river (freshwater) and it is heavily laden with sediment and algae. The water at upper left is less cloudy and is brackish (saltwater). This difference in water density, known as a salt plug, prevents the two waters from mixing, resulting in the dividing line seen down centre. (KEYSTONE/SCIENCE PHOTO LIBRARY/BERNHARD EDMAIER) === ===