(1) Based on the characteristics of the water zone in the Changjiang River and the suspended sediment concentration and salinity, the waters here can be divided into Changjiang runoff, estuarine water, diluted water (further divided into plume water and mixing water outside the estuary) and oceanic water.

(2) The Changjiang Estuary has three salinity fronts From the river mouth to the sea: estuarine front, plume front and ocean front. The estuarine front is the boundary between the estuarine water and plume water, is located outside the estuary, near the water depth of 10m has surface salinity of 5 to 17, and is characterized by large horizontal gradient and the obvious suspended sediments. The plume front is the boundary of the plume water with the mixing water out of the estuary, has surface salinity of 20 to 27, and is characterized by a large horizontal gradient and an obvious vertical stratification. The front is the outer margin of the Changjiang diluted water and has surface salinity of 30 and 31.

(3) The plume front, the most important salinity front in the Changjiang estuary, greatly affects the biology, chemistry, hydrology and sedimentation, and is an estuarine bio-geochemistry zone. Moreover, it also plays an important role in the development of the submerged Changjiang delta and its sedimentation. So it is taken as the outer margin of the Changjiang Estuary.