Stream bio-assimilation capacity is assessed in a developing peri-urban area on the west side of Lyon, France. The total population of Lyon is 1.6 million (2000) and the climate is mild continental with Mediterranean influences. Perennial and intermittent small streams with porous sediment beds and high stormwater and groundwater exchange capacity represent ecosystems very sensitive to pollution. Threats include poor water quality, physical habitat degradation and flood hazard. This study's objective is to adapt Combined Sewer Overflows (CSOs) to the bio-assimilation capacity of the receiving streams (see Figure 9.8). Major actions consist of identifying factors that control bio-assimilation capacity, and exploitating the enhancement of these factors for improving the state of water resources in the catchment.