Droughts have increasingly disrupted the Mississippi River system, which carries roughly half of U.S. grain exports. Using monthly export data for 174 ports from 2013 to 2025, this study examines how the U.S. port network reallocates shipments when river navigation deteriorates. The dynamic causal impact of river disruption on port-level trade flows are estimated using a two-stage local-projection control-function framework, instrumenting grain barge rates with localized hydrological variation across 46 monitoring gauges. Weekly river conditions are aggregated to the monthly frequency using a Mixed-Data Sampling (MIDAS) scheme. We find that the U.S. port network buffers waterway disruptions only in the short-run. Initial buffers temporarily shield Gulf exports, but sustained losses emerge as these margins collapse. Ultimately, the speed and scope of port reallocation hinge upon supply chain diversification and seasonal contractual rigidity.
