The purpose of Ecohydrology is to use natural biomass processes in order to balance local excess of nutrients due to human activities. One application of this principle concerns the impact reduction of combined sewer overflows (CSOs), a mixture of rain runoff with sewer water. During storm events on urban zones, CSOs directly flow into small water courses. Their impact is noticeable on river, because CSO discharges can exceed several times its average natural flow. The objective of the research is to understand and evaluate how an artificial riffle implemented in a small stream can process the pollution discharged by CSOs. As the self-purification depends on time of residence, the first main controlling variable of interest is the hydraulic gradient coupled to the hydraulic conductivity, altogether determining the flux and flow pathways. The presented experiment uses a water pressure sensor network coupled with a multi-parameter probe network, and weekly grab samplings of interstitial waters. Gas emissions are also monitored. Results concern hydraulic gradient variability, monitored in substrate, in conjunction with nitrogen chemical forms, in order to assess the bio-degradation process. Residence time and hydraulic events act both on nitrogen forms and on their concentrations, by the mean of the natural metabolism and dilution factor, depending on stream hydrological status. Perspectives are to design a new experimental pilot to treat as much as possible of the nutrient excess during low flow periods.