The purpose of this PhD. thesis is the modelling of the fate of urban storm-water runoffs discharged through combined sewer overflow (CSO) in a small river. The final aim is to provide an assessment tool of the self-purification capacities of small rivers. The instrumentation of a stream located in the western part of the Lyon's agglomeration has been developed and a protocol for extended hyporheic measurements is actually being proposed. A review of previous results (distribution of benthic and hyporheic invertebrate assemblages, chemical and isotopic analyses) is presented. A good ecological quality occurred at control site (1) and CSO impacted sites show altered invertebrate assemblages (2, 3, 4). At site 3, hydric exchanges between surface and groundwater tend to stimulate the self-purification capacity of the site. The results indicate that the exchanges of water and dissolved substances between surface and subsurface through the hyporheic zone are a key factor when explaining the ecological status of the river. A first stage of system analysis is now beginning.