The water bodies has been suffering with the pollution caused by the disposal of domestic and industrial sewage. The problem is relate, mainly, to the organic matter and scenarios without appropriate treatments become a major problem for the health of society and the environment. Thereby, the present study evaluated the evolutionary framework of the concentration of organic matter in the Tietê river, via mathematical modeling, through values of biochemical oxygen demand. The Tietê river is located in the sub-basin of the Alto Tietê in UGRHI 6, characterized as one of the most polluted rivers in the world and receives large inputs of pollutants by the large number of industries and inhabitants of the region. The model used is based on a system of difference equations and presents, through computer simulations, results that promote relevant discussions in terms of river pollution and indicate that existing discharges, linked to the lack of treatment, act as precursor agent of the quality of the watershed classification and the high levels of pollution in several parts.