The freshwater pulse, characterized by a salinity minimum, has been observed in many coastal areas. A 2-D numerical model was developed to investigate the laterally averaged estuarine circulation, and the freshwater pulse in the St. Lawrence Estuary. The effects on the circulation and salinity of various parameters, including vertical eddy coefficients, river runoff, the bottom friction coefficient and the open boundary condition were studied.The freshwater pulse in the St. Lawrence Estuary was simulated using a seasonal variation of the freshwater runoff. In addition to simulating the downstream propagation and the reduction of the pulse's amplitude toward both the ocean and the deep water in most of the areas, several interesting results were produced. These include (I) the finding of the maximum and minimum amplitudes of the pulse, (2) the increase of the amplitude from surface to deep water in the far upstream region, (3) the initial formation of the pulse at two surface locations, (4) the increase of the arrival time from surface to deep water, and (5) the increase of the arrival time for deep water pulses (at 25 m or deeper) toward the slope region where the upstream shallow water and the downstream deep water separates. The responses of the horizonal and vertical velocities to the freshwater pulse were described. Dynamics associated with the distribution, formation and propagation of the pulse were discussed.