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In estuaries fresh water discharged from rivers meets the salty water of the ocean. At the boundary between these two water masses a front occurs which is called "plume front" or "fresh water front". The front is associated with a narrow band of surface flow convergence where the sea surface roughness usually increases due to hydrodynamic interaction of the surface current with the short surface waves. In this case the front line is imaged by the SAR as a bright line. However, in the presence of surface slicks, slick material accumulates in the convergence zones where it damps the short surface waves. In this case the front line is imaged as a dark line.

Often the river water carries a load of sand and other terrestrial dissolutes which deposit in the estuary. Therefore in most estuaries shallow underwater sandbanks are encountered.

Often the river water is also covered with surfactants, which are either of natural (in most cases) or of anthropogenic origin. They cause a reduction of the short-scale sea surface roughness. In this case the area inside the plume front appears on the SAR image darker than the area outside this front. In addition, the river water usually has a temperature which is different from the temperature of the coastal water. This temperature difference can also give rise to a difference in grey level in the SAR image.

References

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