Influence of river hydrology on the dynamics and water quality of the upper Derwent Estuary, Tasmania

Water quality parameters and current velocities were measured in depth profiles at a series of sampling stations throughout the upper estuary of the Derwent River, Tasmania, during 1988-89. The estuary was severely degraded, with low dissolved oxygen and high sulfide concentrations in benthic saline waters for seven months of the year. A simple two-layer one-dimensional box model was constructed to examine the relationships between water quality, estuarine mixing and river flows. Flow events critical to the dynamics of water quality in the upper estuary are described. An initial analysis of the impact of changes to the flow regime of the Derwent River on the Derwent estuary indicates that hydroelectric storage operations have led to a decrease in the frequency of flows required to flush the estuary, superimposed on climate-induced changes. Recommendations are made for possible flushing flows necessary to improve estuarine water quality.