Dams are supposed to collect water from rivers and redistribute it to alleviate water shortages, right? Not so fast. It turns out that in most cases they actually create water scarcity, especially for people living downstream. Almost a quarter of the global population experiences significant decreases in water availability through human interventions on rivers, says Ted Veldkamp at Vrije University in Amsterdam, the Netherlands. Those interventions primarily involve dams that take water for irrigation or