The purpose of this paper is to evaluate the effect of the proposed deep draft port user fee on export grain flow patterns and provide insight into potential marketing system adjustment costs which may result from diverted flows. A multiperiod, network flow model is used to conduct the analysis. Analyses show grain flow patterns to be affected most by a port specific fee which is based on weight. The annual variation in flows generated by imposition of port user charges is generally less than the historical year-to-year variation and, in most cases, the altered port area flows can be accommodated by existing infrastructure.