Most fixed bed, hydraulic studies in HEC-RAS start with the best, most recent, cross section information available. Sediment studies, however, often start with historic cross sections, and calibrate by running the period of record between these historic cross sections and a contemporary bathymetry. After the model is calibrated, then future projections often start with the best, most recent bathymetry available. But a good set of historic cross sections is one of the most valuable assets a sediment transport study can have.