A numerical model of a salt-wedge estuary developed by Fischer has been expanded and used to calculate the distributions of salinity, temperature, chlorophyll a concentrations, biochemical oxygen demand, and dissolved-oxygen concentrations in the Duwamish River estuary, King County, Wash. With this model, which was calibrated and verified with observed data, computed temperatures usually agreed within 2° Celsius of observed temperatures. During a phytoplankton bloom in the summer of 1968 the computed chlorophyll a concentrations increased and decreased with the observed concentrations; however, during two blooms in 1967 the computed high concentrations persisted farther downstream and lasted a few days longer than the observed concentrations. The computed and observed dissolved-oxygen concentrations usually agreed within 2 milligrams per litre, except during phytoplankton blooms. During the blooms, the differences were often larger, especially when the computed chlorophyll a concentrations were larger than the observed concentrations.
The model was used to predict the dissolved-oxygen concentrations in the Duwamish River estuary when the Renton Treatment Plant sewage-effluent discharge is increased to its proposed maximum of 223 cubic feet per second (6.31 cubic metres per second). The computed monthly average dissolved-oxygen concentrations in the estuary decreased by a maximum of 2 milligrams per litre when compared with computations for the summer of 1971, when the effluent discharge averaged 37 cubic feet per second (1.05 cubic metres per second). The increase in effluent discharge is not expected to cause large changes in phytoplankton concentrations in the estuary.
