The reduction of nutrients and sediments from agricultural runoff by natural wetlands has been commonly accepted, but their role in water quality improvement at the catchment scale has been seldom studied, especially in irrigated catchments. This study aims to elucidate the effect of natural and recently created wetlands on stream water quality after the conversion of a catchment for irrigation purposes. Water quality and morphometrical and vegetation-related variables were measured in 19 wetlands on a 750-ha agricultural catchment under semi-arid conditions in the Ebro basin (NE Spain). A pollution gradient was found, increasing from the wetlands located in the upper catchment to those in the lower catchment. Wetlands with the lowest degree of artificiality, measured as the amount of human created structures (e.g., channel excavation, dikes), and higher plant richness had the poorest water quality, probably because they were in the lower catchment and their water contained more pollutants carried from agricultural and saline soils upstream. Some of these wetlands also had the highest rates of sediment and N-NO₃ retention, in contrast to more artificial wetlands, which exported nutrients and sediments. Less artificial wetlands could also provide ancillary benefits such as biodiversity enhancement or landscape heterogeneity improvement.