Abstract: Operation of cascade reservoirs involves multiple objectives including flood control, hydropower generation and ecosystem protection, and environmental flow demand is usually an important constraint if the major objective is hydropower generation. In this paper, we develop a cascade reservoir operation model for hydropower-environmental flow tradeoff analysis, via a case study of the Lancang River cascade, i.e., the Nuozadu, Jinghong and Ganlanba reservoirs. The results show that an obvious competitive relationship exists between maximization of hydropower generation and minimization of environmental flow alteration ? the more hydropower generation the more alteration of environmental inflow, and vice versa. This relationship is characterized by fairly strong nonlinearity. When alteration index ε is in the range of 0.3 to 0.4, a small change in ε can result in a significant change in hydropower generation, thus showing the importance role of this sensitive window in balance between hydropower generation and biological demand. Moreover, relative to the natural river flow, hydropower operation changes the frequencies of high flow pulses and small floods, which can be improved by an operation rule of artificial flood release.