Flood prediction and mitigation systems are invaluable for improving public safety and community resilience worldwide. Hydrologic and Hydraulic (H & H) simulation of flood events is becoming an increasingly efficient tool for studying and predicting flood events and susceptibility. A consistent limitation of H & H simulations of riverine dynamics is the lack of information about river bathymetry as most terrain data record the water surface elevation over a stream. The impact of this limitation on the accuracy of simulations of the flood has not been well studied over a large range of flood magnitudes and modeling frameworks. Advancing our understanding of this topic is timely, given emerging national and global efforts for developing automated flood prediction systems (e.g. NOAA National Water Center). Here, I study the response of flood simulation to the incorporation of hydraulic geometry derived bathymetry. GSSHA, a distributed 2D hydrologic/hydraulic model with the capability of 1D routing was used as a simulation tool. I test a hypothesis that the impact of inclusion/exclusion of bathymetry data on the model results will vary in its magnitude as a function of river size and flood magnitude. This will allow researchers and stakeholders to better predict flood hazards, benefiting communities in a vulnerable flood zone.