Oxidation of sewage ammonium in the Potomac River is described in terms of a simple kinetic model, with growth of nitrifying bacteria limited by the supply of ammonium ion. The oxidation rate varies inversely with freshwater inflow, and the associated demand for oxygen varies as the inverse square of the freshwater inflow rate. Similar behavior is observed for the Delaware River. The model accounts for the observed concentrations of ammonium and nitrous oxide.