For the biochemical study of assimilation capacity (AC), two hypotheses were put forward (three levels of response, the hyporheon being the main site of self-purification). A physical model was designed, simulating the hyporheon and its response to increasing inputs of proteins was studied. The analyses evidence a low accumulation of proteins in the hyporheic biofilms for daily flows ranging from 0.1 to 20 mg/day. The percolation of proteins stimulates the proteolytic activity (increase in Vm) whereas the Km shows no significant change; the assimilation capacities thus stimulated are vastly superior to the daily loads introduced into the system and explain the low accumulation of proteins. This stimulation concerns only a shallow active zone < 2 cm. For the future, a method of localising zones of hyporheic infiltrations (micro-piezometry, tracing, etc.) should be developed to monitor in situ the action of inputs of organic matter (OM) to estimate the AC of the river.