Characteristics of the physical determinants were defined by using the premise that in porous media the bio-assimilation capacity depends on the active exchanges between surface and groundwaters (Jones and Mullholland, 2000; Hancock, 2002; Hancock and Boulton, 2005; Hancock et al., 2005; Breil et al., 2007). In the case of the Yzeron streams, the hyporheic corridor is several metres wide and not more than a metre deep. This corridor is considered to be the major site for degradation processes in the river.