A new distributed periphytic model has been developed and implemented in the hydro-biological river model PS, which models the biogeochemical processes in rivers. The transient periods, like floods, are taken into account, and periphyton scouring is represented in an original way. A validation step shows agreement between field data (carbon and nitrogen) concerning either the water column or the periphyton and outputs of the model. The first results have shown that the decreases of ammonium concentrations along the modeled stretch (40 km) are mostly driven by nitrification in the periphyton, the other processes being of smaller importance on the ammonium balance. Then, the model was used to extrapolate the punctual benthic estimations to the whole river and to determine carbon fluxes at the scale of the studied stretch. The carbon biomass, which is fixed on the bottom of the stretch, can reach several tons depending on the development stage of the periphyton. In case of flood, the exported flux of this biomass can reach 1.4 t C day