A sophisticated and one-dimensional model to simulate the concentration of PO 4;P (x,i) in a shallow and polluted river is presented; the model incorporates, other than the convection and dispersion, various physico-chemical and biochemical reactions of phosphorus sinks and sources. With reference to the field data on Tama-gawa, which penetrates through the metropolitan area of Tokyo, the model is confirmed to represent the concentration of PO 4;P fairly well in the mid-region except for the mountainous origin and its estuary. A material balance of PO 4;P, the calculation of which is made possible from the model, reveals that about 15% of the daily phosphorus input into the region is fixed by algae on the river bottom, while about 54% of the input flows downstream without being fixed. The balance also discloses that phosphorus decrease due to adsorption on to suspended solid and the increase attributable to hydrolysis of condensed phosphates in the water can be disregarded, respectively. The rest of PO 4P balance amounting to about 30% is composed of adsorption on to the river-bed, seepage into the groundwater and release from the non-viable algal decomposition. The model is also used to simulate the PO 4;P concentration in the region under various conditions. Though naive, the most effective means to decrease the concentration is found, from the model, to be to curtail as much as possible the phosphorus inflow from tributaries.