The hydrodynamics of river mouths are the result of a complex interaction between river flow, tidal conditions and outlet geometry. This complex interaction of factors shapes the jet that flows onto the continental shelf and influences the dynamics of these areas. To gain insight into the response of the jet to different outlet and nearshore geometries and changing river discharge and tidal conditions, the hydrodynamics of idealised river mouths are simulated numerically. Compared to previous work, the model includes transient river discharge and tidal conditions and more realistic nearshore geometries. Comparison with classical jet theory indicates that the model adequately simulates jet hydrodynamics. The results show that both the outlet geometry and the transient river discharge and tidal conditions have a significant influence on the jet structure and evolution along the nearshore. For constant river discharge and water level conditions, the results indicate that the nearshore profile plays a key role in determining the expansion or contraction of the jet. The momentum balance shows that the jet behaviour is related to the momentum transport and the barotropic terms. In cases where the river discharge and tidal conditions are transient, the jet alternates between a structure with two velocity maxima at the edges or a single peak in the centre during the tidal cycle. This alternation occurs as a function of the time along the tidal phase and the time lag between the tidal conditions and the river hydrograph. The morphodynamic consequences of these two different jet structures are also discussed.