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We compared the nitrate removal efficiency of two constructed wetlands receiving ambient river water to one constructed municipal wastewater treatment wetland over the same 2-year period in central Ohio, USA. The wastewater wetland represents a high-nutrient system, with an average nitrate plus nitrite load of 12.3 kg N ha^-1day⁻¹ and an average nitrate and nitrite inflow concentration of 12.5 mg N l⁻¹. The riverine wetland loadings and concentrations were approximately 60% lower (4.6–4.7 kg N ha⁻¹day⁻¹ and 4.6 mg N l⁻¹). Percent nitrate removal by mass ranged from 29% in the wastewater wetland to 37–40% in the riverine wetlands, although differences in retention varied widely by season and were not statistically significant among the wetlands. Retention efficiency was considerably lower in all three wetlands during floods; nitrate outflow was as much as 400% greater than inflow during some flood …