The present work describes the development and calibration of a mathematical model for the Tigris River downstream of Sadam Dam. The river stretch studied is 75 km long extending from the Sadam Dam to Mosul city. The field work was conducted during the period from July to September 1986. Water samples were collected bimonthly from specified sampling points. The model simulates river assimilation capacity for a variety of water quality parameters by performing the numerical solution of a set of differential equations representing the aquatic system under steady state conditions. Generally, a noticeable increase in the concentrations of water quality parameters arising from water impoundment was observed. A good agreement was found between measured and simulated concentrations of water quality parameters. However, discrepancies noticed during model calibration were attributed to the assumptions adopted in the model formulation, to lack of field data, and to exclusion of some variables in model building.