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 its 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 tid