This paper assesses the impacts of the Three Gorges Dam, the South-North Water Transfer Project and other water abstractions on the probability of long-duration salt intrusions into the Yangtze River estuary. Studies of intrusions of saltwater into estuaries are typically constrained by both the short duration of discharge records and the paucity of observations of discharge and salinity. Thus, studies of intrusions of saltwater into estuaries typically seek to identify the conditions under which these intrusions occur, using detailed observations for periods of 20-60 days. The paper therefore first demonstrates a method by which to identify the conditions under which intense intrusions of longduration occur and then applies that method to analyse the effect of the three projects. The paper constructs a model of the relationship between salinity and discharge and then employs Monte Carlo simulation methods to reconstruct the probability of observing intrusions of differing intensities and durations in relation to discharge. The model predicts that the duration of intrusions with chlorinity ≥ 250 mg L −1 (or ≥ 400 or 500 mg L −1 ) increases as the number of consecutive days with discharge ≤ 12 000 m 3 s −1 (or ≤ 8000 m 3 s −1 ) increases. The model predicts that in 1950-2014, the number of consecutive days with chlorinity ≥ 250 mg L −1 averaged 21.34 yr −1 ; if the three projects operate according to their normal rules, that average would rise to 41.20 yr −1 . For a randomly selected year of discharge history from the period 1950-2014, under normal operating rules for these projects the probability of an intrusion rises from 0.25 (for 30-day intrusions) or 0.05 (for 60-day intrusions) to 0.57 or 0.28, respectively.

Abstract. Studies of intrusions of salt water into estuaries are typically constrained by both the short duration of discharge records and the paucity of observations of discharge and salinity. Thus studies of intrusions of salt water into estuaries typically seek to identify the conditions under which intrusions occur, using detailed observations for periods of 20–60 days. This paper demonstrates a method by which to identify the conditions under which intense intrusions of long duration occur and applies that method to the Yangtze River estuary. The paper constructs a model of the relationship between salinity and discharge and then employs Monte Carlo simulation methods to reconstruct the probability of observing intrusions of differing intensities and durations in relation to discharge. The model predicts that the duration of intrusions with chlorinity ≥250 mg L−1 increases as the number of consecutive days with discharge ≤12 000 m3 s−1 increases; consecutive days of discharges ≤8000 m3 s−1 predict the duration of intrusions with chlorinity ≥400 or 500 mg L−1. In 26 of the 64 years analysed, the probability of an intrusion of at least 60 days at ≥250 mg L−1 is greater than 1 in 1000; in 17 years is greater than 1 in 100; and in ten years is greater than 1 in 10.