The Columbia Wastewater Treatment Wetland (“Columbia Wetland”) is a constructed cattail wetland in the Missouri River floodplain outside Columbia, Missouri, USA. The wetland receives mixed primary and secondary effluent (≈60,000 m³ d⁻¹, BOD₅ ≈30 mg L⁻¹, TSS ≈13 mg L⁻¹, NH₄-N ≈8 mg L⁻¹) from a conventional treatment plant. During its first 6 years of operation (October 1994 through November 2000), the wetland received loadings of BOD₅, COD, and NH₄-N averaging 50, 83, and 12 kg ha⁻¹ d⁻¹, respectively, of which averages of 74%, 30%, and 17%, respectively, were removed from the effluent. TSS (mean loading = 21 kg ha⁻¹ d⁻¹) frequently increased in the wetland due to erosion and disturbance by waterfowl, but TSS removal efficiency increased with time and the development of macrophyte biomass and averaged 30% by 1998. The wetland typically removed >97% of fecal coliforms and fecal streptococcus, 36% of TN and 4% of TP. In comparison to other large treatment wetlands, BOD removal by the Columbia Wetland has been exceptionally efficient.