Abstract: Nonlinear behaviors in flood routing of a reservoir system have been analyzed to study the intrinsic stochasticity of the system, aiming to form a basis of uncertainty and risk analysis for reservoir flood control systems. Applying the system dynamics theory and system simulation method, we examined the nonlinear dynamic characteristics of a deterministic mathematic model for reservoir flood routing of water level and outflow under the conditions of free overflow and regulated outflow. The results show that the state variables were not sensitive to the initial disturbance in the stage of free overflow while quite sensitive in the stage of regulated outflow. In the latter case, water level and outflow were evolving in a way of amplified deviations as a response to the initial disturbance of the routing. This phenomenon indicates that nonlinear behaviors in reservoir flood routing could play an important role in system uncertainty and it is a potential risk factor and an underestimated problem. Thus, it should be further studied owing to its importance associated with the interests and risk in reservoir flood control.