This paper investigates the effect of reservoir operation on downstream water quality. This is accomplished by the following procedures. An automated channel network extracting system, C-NES, is used to extract channel network from Digital Elevation Model and a distributed rainfall-runoff model is adopted to estimate the streamflows. The relationship between water quality and streamflows is established Subsequently, a reservoir water quality model, WASP, is employed to predict water quality under several sedimentation conditions and operation policies. Different reservoir operation rules are simulated to address the possibility of maintaining water quality through proper operation. The watershed of Ta-Chi Reservoir in Taiwan is chosen as a case study. The simulation results show that an appropriate operation rule can improve outlet water quality by releasing water from a certain depth outlet in the reservoir. This is because the flow field that influences the bio-chemical reactions is altered. It is suggested that careful consideration of the outlet positions should be included in the future design and operation of a reservoir. Copyright (C) 1996 Elsevier Science Ltd.