The coevolution of a meandering channel and its floodplain comprise a complex web of biomorphodynamic feedbacks and associations where channel flow is the forcing process and the floodplain serves as storage and memory component of the system. Differences in erodibility and flow connectivity throughout the floodplain serve to modify the morphology of the meandering river at various scales. Informed by field-based studies, modeling efforts have tried to understand the governing feedbacks and biophysical factors (e.g., sediment size, geology, or vegetation communities) with regard to flow variability, sediment transport, and floodplain erodibility. In this work we aim to contribute to the knowledge-base associated with these complex feedbacks by developing a long-term biomorphodynamic river-floodplain coevolution model. We use this model to determine the most impactful biophysical factors by conducting a structured sensitivity analysis that assesses impacts of these biophysical factors to form and function within the channel-floodplain system. Specifically, we evaluate biophysical factors in three groups: (1) flow/sediment transport, (2) floodplain topography, and (3) floodplain vegetation. By evaluating biophysical factors through the lens of these groups, this work aims to help researchers and stakeholders better understand the most impactful biophysical factors that should be considered for studying the long-term characteristics of a given river-floodplain ecosystem.