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Nitrate-nitrogen retention as a result of river water diversions is compared in experimental wetland basins in Ohio for 18 wetland-years (9 years×2 wetland basins) and a large wetland complex in Louisiana (1 wetland basin×4 years). The Ohio wetlands had an average nitrate-nitrogen retention of 39g-Nm⁻²year⁻¹, while the Louisiana wetland had a slightly higher retention of 46g-Nm⁻²year⁻¹ for a similar loading rate area. When annual nitrate retention data from these sites are combined with 26 additional wetland-years of data from other wetland sites in the Basin Mississippi River (Ohio, Illinois, and Louisiana), a robust regression model of nitrate retention versus nitrate loading is developed. The model provides an estimate of 22,000km² of wetland creation and restoration needed in the Mississippi River Basin to remove 40% of the nitrogen estimated to discharge into the Gulf of Mexico from the river basin. This …