Unusual Salinity Conditions in the Yangtze Estuary in 2006: Impacts of an Extreme Drought or of the Three Gorges Dam?

During the extreme dry year of 2006, abnormal salinity conditions in the Changjiang Estuary of the Yangtze River occurred in partial coincidence with the second impoundment phase of the TGD (Three Gorges Dam). Analysis of discharge observations in the upper reaches of the estuary and of salinity observations in the estuary as a whole reveals that in 2006 salinity was over 100 mg/l during 275 days, over 250 mg/l during 75 days and over 400 mg/l during 48 days. It is well known that this is due to extreme low discharges from the upper catchment area into the estuary. Moreover, large amounts of water consumed along the lower reaches of the Yangtze River can also aggravate the low discharges that lead to stronger saltwater intrusion in the estuary. Of the 75 days that salinity was over 250 mg/l, the low discharge was decreased further by 10 to 20% due to water consumption. The additional impact of the impoundment phase of the TGD (lasting 37 days in autumn) was noticeable only during 7 days in 2006. During that period, the relative contributions of the TGD and the water consumption in the lower reaches of the Yangtze River amounted to 70 and 30%, respectively. It may be concluded that the impact of the second impoundment phase of the TGD on salinity intrusion in the estuary was modest, while the extreme drought of 2006 was the dominant cause.