Assume that the stream velocity is constant throughout the reach of interest. Algal respiration never causes the dissolved oxygen concentration to exceed the saturation value. Oxidation of NBOD is subject to retardation by low dissolved oxygen concentrations. The rearation coefficient is most commonly a function of temperature, stream velocity, stream slope and stream depth. In the absence of sources and sinks, the concentration of a conservative substance does not change along the length of a river.
Dispersion is the least important mixing process in river modeling.
The Thomas method is used to estimate CBOD settling rates. CBOD and NBOD can be combined into a single pollutant term when the settling rates are equal. Deep rivers are most susceptible to effects of sediment oxygen demand.