|Salt intrusion has large consequences for estuarine environments and communities. Salinization harms the ecology and biodiversity in an estuary and it might constrain the possibilities of using water for irrigation, industry and human consumption. Due to the seasons, climate change and increasing anthropogenic influences, the river discharge might change on short or long timescales which will affect the salt intrusion and the morphology on the long term. Previous research has been done to the effect of river discharge on salt intrusion, but those use a morphostatic model with a constant width profile. In this thesis, a one-dimensional tide-averaged numerical hydrodynamic and morphodynamic model is used, which includes a variable width and a constant bed level. This model is best suitable to simulate the response of estuaries globally, and less suitable to look into details for individual estuary systems. The estuary width is calculated by an optimized empirical hydraulic geometry relation that relates the width to the river discharge and tidal peak discharge. River discharge and tides force the upstream and downstream boundaries, respectively. For the salt intrusion length, an empirical method was used after validation. On the short term, the morphology of an estuary will not change, meaning that only the hydrodynamics can affect the salt intrusion length. This results in higher salt intrusion lengths for lower river discharges, since the tide is less countered by friction with the river flow. On the long term, the estuary will narrow for smaller river discharges, which causes more friction between the tide and the banks causing a lower salt intrusion. For some estuaries, the salt intrusion length increases for river discharges above the yearly averaged discharge, since friction diminishes. River-dominated estuaries, where the river discharge is much larger than the tidal discharge, experience much more narrowing. It becomes difficult for the tide to enter the estuary causing a large reduction in salt intrusion length on the long term. River discharge has a small influence on the salt intrusion length and morphology of tide-dominated systems. Mixed systems have a large increase in salt intrusion when the river discharge decreases on both the short and the long timescale, since widening of the estuary mouth causes that the tide can still easily enter after narrowing. This 1D numerical model can be used for further research when the goal is to gain insight into the long term evolution of estuaries, for example when the effects of sea-level rise or sediment deficits are investigated.