A hydrodynamic model and a water quality model for the Duluth-Superior harbor are developed and verified by using data on currents, water levels, and water quality parameters. The water quality model is subsequently used to simulate the transport of particulates resuspended by ship traffic and the dispersal of dissolved material spilled into the harbor at a major industrial site. Resuspension of bottom sediments by ship traffic is an important secondary source of harbor turbidity. Suspended solids in ship resuspension plumes range from 10 to 50 mg/l, five times the usual concentration of suspended solids in the harbor. An estimated 100,000 kg of material is resuspended per passage of a ship. The resuspended material is coarse and settles rapidly, thus only 0.1% of the material flows out directly into Lake Superior. It was found that a spill of dissolved pollutant in the inner harbor would take from 8 to 21 days to reach Lake Superior at peak concentrations.