A mathematical model of phosphate release rate from sediment, fp, is presented which determines the fp as a function of flow velocity over the sediment and dissolved oxygen concentration. Oxygen consumption in the sediment is expressed as the sum of chemical consumption due to ferrous iron oxygenation and the bacterial consumption which is assumed to be a first order reaction of oxygen. At very low flow velocities, transport through the diffusive boundary layer is the limiting factor of SOD, and phosphate release rate is expressed as a linear decreasing function of the velocity. When flow velocities are increased, both SOD and phosphate release rate become independent of velocity, since the reactions in the sediment are the rate limiting factor. The model suggests that phosphate release flux is a linear decreasing function of DO in the bulk water, while SOD is an increasing function of DO concentration. The critical DO concentration at which the phosphate release ceases is expressed in terms of the flow velocity. The prediction of SOD and ϕp by the present model is favourably compared with experiments by former researchers.