Abstract River waters downstream of a hydroelectric project are often subject to rapidly changing discharge. Abrupt decreases in discharge can quickly dewater and expose some areas and isolate other areas from the main river channel, potentially stranding or entrapping fish, which often results in mortality. A methodology is described to estimate the areas dewatered or entrapped by a specific reduction in upstream discharge and applied in a case study. A one-dimensional hydrodynamic model was used to simulate steady flows. Using flow simulation results from the model and a geographic information system (GIS), estimates of dewatered and entrapped areas were made for a wide discharge range. The methodology was applied to the Hanford Reach of the Columbia River in central Washington State. Results showed that a 280 m 3/s discharge reduction affected the most area at discharges less than 3400 m 3/s. At flows above 3400 m 3/s, the affected area by a 280 m 3/s discharge reduction (about 25 ha) was relatively constant. A 280 m 3/s discharge reduction at lower flows affected about twice as much area. The methodology and resulting area estimates have been used to identify discharge regimes, and associated water surface elevations, that might be expected to minimize adverse impacts on juvenile fall chinook salmon ( Oncorhynchus tshawytscha) that rear in the shallow near-shore areas in the Hanford Reach.