The purpose of Ecohydrology is to take as much as possible opportunity of the natural biomass processing to balance the local excess of nutrients produced by human activity. One application of this principle concerns the management of combined sewer overflows (CSOs) from sanitation systems. During storm events, CSOs are directly flowed to the natural receiving media, often into water courses. Their impact is noticeable since the ratio of CSOs to natural flow is of 0.5 to more. This often occurs for small water courses, tributaries of rivers that spread in periurban areas. The impact reduction of CSOs was identify to be a priority in the USA water act and was pointed out during a European research project dedicated to inter-calibration of bio-indicators. The objective of the presented research is to compare natural and enhanced self-purification in a small seasonal water course that can dry up for 3 to 4 months a year. In that situation and in combination to a reduction of CSOs it is looked to the effect of constructed porous steps aiming at increase the polluted flow transfer into the natural porous sandy substrates of the creek. The natural and constructed reaches are located just downstream a CSO device that influences the natural flow of water during overflow events. 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. Results of hydraulic gradient monitored in the substrates and hydraulic simulations are presented both for a natural riffle and a constructed step. The natural reach shows a more variable interstitial flow of water controlled by the discharge when the constructed step has a more regular flux of interstitial water. Time of residence are clearly controlled by the creek flow but overall, interstitial water stays more longer in the constructed riffle. It can provide a more efficient self-purification by mineralization of the organic matter. In the perspective, to process the amount of point pollution delivered during storm events the porous step system should be repeated downstream to figure a sequence of small cascades. Three steps are now implemented.