Beyond the river mouth it increases rapidly. Salinity in the vicinity of Qiyakou in the south branch is higher than either upstream or downstream. The same situation exists in the Xinqiao channel. This is the result of the backing up of saltwater from the north branch into the south branch. Blocking up the north branch would have the great advantage of improving the navigation channel in the south branch and the drawing of freshwater for Chongming Island.
Salinity depends mainly upon the distribution of runoff.
Lower discharges mean lower salinities and vice versa. In a multiple channel, the salinity of a given cross-section is higher in flood tide than in ebb. At the beginning of the flood tide within one of the channels, the salinity is higher at the sides than in midstream, while at full flood and flood slack, midstream salinity is higher than along the banks. At full ebb, side salinity is again higher than at midstream. Salinity along the north bank is higher than along the south bank.