Flood wave propagation modeling is of critical importance to advancing water resources management and protecting human life and property. In this study, we investigated how the advection-diffusion routing model performed in flood wave propagation on a 16 km long downstream section of the Big Piney River, MO. Model performance was based on gaging station data at the upstream and downstream cross sections. We demonstrated with advection-diffusion theory that for small differences in watershed drainage area between the two river cross sections, inflow along the reach mainly contributes to the downstream hydrograph’s rising limb and not to the falling limb. The downstream hydrograph’s falling limb is primarily determined by the propagated flood wave originating at the upstream cross section. This research suggests the parameter for the advectiondiffusion routing model can be calibrated by fitting the hydrograph falling limb. Application of the advection diffusion model to the flood wave of January 29, 2013 supports our theoretical finding that the propagated flood wave determines the downstream cross section falling limb, and the model has good performance in our test examples. Flood wave propagation modeling is of critical importance to advancing water resources management and protecting human life and property. In this study, we investigated how the advection-diffusion routing model performed in flood wave propagation on a 16 km long downstream section of the Big Piney River , MO. Model performance was based on gaging station data at the upstream and downstream cross sections. We demonstrated with diffusion-diffusion theory that for small differences in watershed drainage area between the two river cross sections, inflow along the reach mainly contribute contribute to to downstream hydrograph’s rising limb and not to the falling limb. The research suggests the parameter for the advection diffusion routing model can be calibrated by fitting the hydrograph falling limb. Application of the advection diffusion model to the flood wave January 29, 2013 supports our theoretical finding that the propagated flood wave determines the downstream cross section falling limb, and the model has good performance in our test examples.