A simple and efficient procedure for incorporating the effects of streamchannel geometry in the distributed modeling of catchment dynamics is developed. At-astationand downstream fluvial relationships are combined and the obtained laws ofvariability in space and time for water-surface width and wetted perimeter areincorporated into a diffusion wave routing model based on the Muskingum-Cunge methodwith variable parameters. The parameterization obtained is applied to the approximately840-km2 Sieve catchment (Central Italian Apennines) to test the possibility of estimatingchannel geometry parameters from cross-section surveys and to assess the impact ofdynamic variations in the channel geometry on catchment dynamics. The use of theestimated channel geometry in surface runoff routing produces a significant improvementin the flood hydrograph description at the catchment outlet with respect to less detailednetwork parameterizations. In addition, the results obtained from a “downstream” analysisof the velocity field indicate that the stream characteristics related to the locally varyingcross-section shape may have a strong control on flow velocities, and thus they should bemonitored and synthesized for a comprehensive description of the distributed catchmentdynamics.