Field data gathered from a Western Australian reservoir are used to obtain an estimate for the entrainment coefficient of the underflowing river. By the use of a steady normal flow theory, the entrainment coefficient is related to the flow and river channel characteristics. By combination with a general turbulent entrainment law, it is shown that the entrainment coefficient and dilution may be directly related to the physical characteristics of the river channel, and thus may be estimated without recourse to a field experiment. In order to determine the initial underflow depth and the location of the plunge line, a simple theory using the concept of a normal flow depth is used, the results of which compare favorably with observation.