On July 19, 1980, 80% of the runoff of Eastmain River, a subarctic shallow estuary, was diverted into the La Grande River for hydroelectric development. Consequent to the diversion the estuary, which had been mostly salt-free, was subjected to a gradual saltwater intrusion along a 10-km section in it lower reaches. The adjustment of the salinity regime to a new quasi-steady state took place over a period of about 40 days. The circulation field reached a new quasi-equilibrium within 8 days. The 2-month interval following the river diversion, termed the transition period, was analyzed in terms of both time and space modifications of the circulation and salinity fields. A one-dimensional finite difference explicit scheme numerical model was applied to the estuary as a complement to observational data. Good agreement was obtained between the model results and the low-frequency current meter observations of salinity and longitudinal velocity. These results showed that the salinity intrusion was primarily governed by tidal dispersion.