Larger US river–floodplain ecosystems are severely degraded. We analyze how public proprietary interests in streambeds and floodplains afford enormous untapped opportunity to protect them. We estimate that states hold in trust for the public approximately 500,000 kilometers of streambeds and hundreds of thousands of hectares of island-derived floodplains that continue to form at thousands of hectares annually. We find that although courts in 42 states have enforced public proprietary interests in submerged lands and floodplains, only three states have inventoried public streambeds, and no state has a comprehensive program to find, claim, and manage public streambeds and floodplains. We describe a legally and scientifically sound strategy to limit human interference with fluvial geomorphic processes, thereby regenerating diverse habitats and securing their myriad benefits, and we show how numerous successes in claiming and protecting public submerged lands and floodplains in dozens of states confirm the validity and power of this strategy.