Reservoirs, driven jointly by artificial regulation and natural processes, are semi-artificial complex ecosystems whose equilibrium states affect the water ecological security and management effectiveness of a river basin. Understanding how operational dispatch alters hydrodynamic processes, habitat structures, and biological succession pathways is necessary, as traditional steady-state theories based on natural lakes struggle to fully explain the responses of reservoir ecosystems. Therefore, this review constructs a theoretical framework for reservoir ecosystem regimes, defining the core connotation of their “conditional metastable state” as the long-term maintenance of physicochemical, biological, and hydro morphological parameters within limits that allow ecosystem functioning under anthropogenic regulation and natural disturbances. It identifies typical characteristics such as habitat reconstruction-driven dynamics, altered ecological resilience, dual threshold effects, and regulation-modified hydrological processes. We propose that reservoir regime shifts can be analyzed from three levels: the evolutionary sequence of the full life cycle, stage transitions during the operational period, and disturbance responses at multiple time scales.