A three-dimensional, unstructured grid, hydrodynamic model was established and applied to study the salt water intrusion and residual circulation in the Danshuei River estuarine system of northern Taiwan. The construction of two reservoirs (i.e. Shihmen Reservoir and Feitsui Reservoir) and water diversion in the upper reaches of the river system significantly reduces the freshwater inflow. The changes had contributed farther to the intrusion of tidal flow and salt water in the upstream direction. The model was validated with available hydrographic data measured in 2001 then used to probe the change in salt water intrusion as a result of reservoir construction. The model simulations indicate that more tidal energy propagates into the estuarine system after reservoir construction because of the substantial increase in river crosssections and decrease in freshwater discharges. The residual circulation before reservoir construction is weaker than that after reservoir construction. The limits of salt water intrusion after reservoir construction extend farther inland 3.5 km than those before reservoir construction under mean flow condition. This case study offers the quantitative estimate of the salinity and residual circulation changes due to human interface in this nature system.

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