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River deltas occupy only ~0.65% of Earth's land surface, but collectively house ~4.5% of the global population and account for more than 6% of the global GDP. Because of ongoing human interventions in the past century (river diversions, groundwater and petroleum extraction, and urbanization), deltas are coming under additional and intense threat from climate change and the impending sea-level rise. Many highlatitude and tropical deltas where population pressure is low and human modification is minimal face less peril for the foreseeable future, but densely populated deltas, especially those in Asia with extreme urbanization and environmental pressures, will be more susceptible to land loss and drowning. Here we consider six key deltas--Mississippi, Yangtze, Niger, Bengal, Nile, and Indus--emphasizing recent findings and consensus as to their health and how human activities have brought these vulnerable ecosystems perilously close to or beyond the point of no return. [ABSTRACT FROM AUTHOR]