A new, unsteady analytical solution is presented to simulate the spatio-temporal variation in salinity in convergent, well-mixed estuaries. A first application in the Pearl River estuary shows it to be an efficient approach for predicting salt intrusion dynamics with satisfying accuracy. The model can be used to decide on water-fetching methods related to drinking-water supply. The model explains and quantifies the time difference between salinity at high water slack and the maximum salinity.