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This paper studies the effects of the implementation of wastewater treatment (WWT) on the water quality of small urban river systems by considering as an extreme case study (volumetric contribution of wastewaters >50 %) the evolution of the Zenne River waters (Belgium) over the last 40 years. In urban rivers, organic matter (OM), oxygen, and nutrients are primarily controlled by wastewater releases which depend on the population and the WWT capacity in the river basin, the latter being dependent on environmental policy decisions. We introduce a novel basin-scale evaluation method that considers the evolution of annual pollutant loads at the outlet of the river basin directly as a function of WWT capacity. Based on this approach, we could prove that the load reductions observed after the implementation of WWT in the river basin was a good indicator of the global treatment efficiency of the WWT plants. We …