River channels and their flood plains are among the most naturally dynamic ecosystems on earth, in large part due to periodic flooding. The components of a river's natural flood regime (magnitude, frequency, duration and timing of peak flows) interact to maintain great habitat heterogeneity and to promote high species diversity and ecosystem productivity. Flood regimes vary within and among rivers, depending on catchment size, geology and regional hydroclimatology. Geographic variation in contemporary flood regimes results in river–to–river variation in ecosystem structure, and therefore in potential river ecosystem response to increased future flooding. The greater the deviation in flood regime from contemporary or recent historical conditions, the greater the expected ecological alteration. Ecological response will also depend on how extensively humans have altered natural river dynamics through land–use practices. Examples of human–caused changes in flood regime (e.g. urbanization, agricultural practices) provide analogues to explore the ecological implications of region–specific climate change. In many settings where humans have severely modified rivers (e.g. through leveeing), more frequent larger floods will work to re–establish connections with severed flood–plain and riparian wetlands in human–dominated river valleys. Developing and implementing non–structural flood–management policies based on ecological principles can benefit river ecosystems, as well as human society.