Over the past several decades there has been a gradual tendency for the number of low flow years to increase. Data collected at the Hankou station between 1956 and 1974 show nine low flow years and only two high flow years. Saltwater intrusion in the estuary has worsened. Because of consecutive years of excessively low flows, salinity at the Yinshuichuan station in 1978 was about 0.20 per cent greater than the long-term average. He estuary to 125 E longitude.
Salinity increases gradually downstream.
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.