An investigation of the Quashnet River stream- aquifer system on Cape Cod was initiated in response to concern over possible streamflow reduction and degradation of the sea-run brown trout habitat of the river resulting from proposed ground-water withdrawals. A two-layer finite-difference ground-water-flow model was developed to simulate the stream-aquifer system. Steady-state pumping rates of 0.5, 1.0, and 2.0 million gallons per day were simulated at three well sites 1,500 to 2,500 feet west of the river. No infiltration of water from the river to the aquifer was induced in any of the simulations. Maximum streamflow depletion along the river for the scenarios tested ranged from 3 to 15 percent of calculated steady-state prepumping streamflow. Mean monthly streamflow depletions determined by use of the transient model, for a constant withdrawal of 1.0 million gallons per day from a site 1,500 feet west of the river, range from 6 to 8 percent of the mean monthly streamflows measured at a gage located 0.3 miles from the mouth of the river. A particle-tracking postprocessor to the steady-state model was used to delineate contributing areas of the river and the proposed withdrawal sites. Although the simulated cone of depression produced by pumping extends beyond the river, the contributing area of the well does not include the river under any of the withdrawal schemes simulated.