Around the world many river cities are challenged simultaneously by heightened flood risk and degraded river health. The prevailing approach to flood hazard mitigation--flood control, has been criticized to be ineffective for long-term flood safety. Flood control is also well-understood to be destructive to river-floodplain ecosystems through the homogenization of the biophysical environment and elimination of periodic flooding that maintains ecological functions. Despite these recognitions, it is still widely believed that flood control is indispensable in cities and that the associated ecological degradation is a necessary tradeoff for flood safety. This dissertation research challenges such conventional wisdom by understanding the dynamics of river cities as coupled human-natural systems whose system-level properties arise from the interactions of human and riverine processes, and by providing a theory for alternative urban flood hazard mitigation. This research is composed of three interrelated investigations that are presented as three independent chapters. The first chapter integrates disciplinary knowledge associated with flooding to provide an interdisciplinary account of the complex dynamics of human-nature couplings in cities dependent on flood-control infrastructure. Issues of flood control as a simple solution to a complex problem are discussed. The second chapter develops a theory of 'urban resilience to floods' as a theoretic framework for alternative flood hazard management to better respond to the inherent dynamics and complexity of river cities. I explain why flood control is in fact dispensable and argue for a paradigm shift from flood control to flood adaptation to create resilient "floodable" cities. The last chapter then links flood safety to river health, exploring the practical solutions to and demonstrating the ecological benefits of floodable cities, where periodic floods are accommodated rather than resisted. Overall, this dissertation research provides a new way of approaching urban flood hazards. It serves as a point of departure for developing design and planning theories and practices that explicitly address the interactions of the urban built environment and inherent environmental dynamics.