In this paper, we investigate the effects of the bathymetry of the floodplain distributaries using a 2-dimensional hydrodynamic model of the Wax Lake Outlet (WLO), delta and floodplain (Louisiana, USA), using Delft3D. Modelling the tidal-fluvial interaction of this region is challenging because of its complex network of low-lying floodplain distributaries and vegetation coverage. This investigation addresses this interaction by generating the evolution of the flood-map that is used in the calibration and validation phase of the Surface Water and Ocean Topography (SWOT) mission. Accordingly, the model is set up for one week of calm conditions from October 14th to 20th in 2016. Boundary conditions applied at the edges of the 36 × 23 km ² domain include the offshore water level with tidal oscillations, the upstream volumetric river discharge rate and wind time-series (applied spatially uniformly). The topo-bathymetry of the domain is a product of the Pre-Delta-X mission; however, it has considerable uncertainties to represent the small distributaries, especially in the forested section of the region. This paper tackles this uncertainty by parametrically enhancing and calibrating the primary Digital Terrain Model (DTM). The performance of the model is assessed using the water-level time series measured by the stations spread throughout the floodplain, delta and main river. The results show that the morphology of the distributaries play a major role in the hydrodynamic of Wax Lake region under calm conditions. The hydrodynamic of the WLO and floodplain is considerably affected by the discharge capacity of the distributaries (changed by editing their width and depth) due to change of the balance between roughness and topographic convergence. In WLO, higher discharge capacity leads to higher overall roughness friction force, thereby dampening the amplitude of the water-level signal and vice versa. On the other hand, increasing the bottom roughness of the WLO increases the water level in the floodplain, with more considerable effect in the low tides. The calibrated model well matches the measurements along the Wax Lake Outlet. However, the main discrepancies are in the stations further away from the WLO. The tests conducted in this research suggest that the excessive roughness due to uneven bed along the distributaries and vegetation are, respectively, the potentially-responsible parameters for the remaining discrepancies in low- and high-tide periods.