A systematic methodology for mathematically simulating photosynthetically driven DO variations in the mid‐Passaic River system is presented and discussed. A steady‐state model for BOD‐DO dynamics, calibrated and verified to data from two stream surveys conducted during the Summer and Fall of 1983, is first presented. Diurnal DO fluctuations due to algal photosynthetic activity are then simulated by Fourier series. The resultant models display good correlation to the data.