Pesticides are often produced and stored in large quantities near rivers posing a potential hazard for the aquatic environment. Accidental incidents such as storage facility fires are of major concern as significant amounts of pesticide chemicals can enter the nearby riverine system possibly causing considerable environmental damage (for example the Sandoz fire in Basel in 1986 which polluted the Rhine river). In order to assess the potential impact on the riverine environment of such an incident it is necessary to identify the controlling environmental fate processes that affect pesticides in freshwater systems. A strategy for mathematically modelling these processes to predict the fate of the chemical contaminants then needs to be established. This paper discuss and reviews the major environmental controlling fate processes of selected herbicides in freshwater. Strategies on how to mathematical model the environmental fate of pesticides in rivers are also considered.