Rivers are crucial components of human well-being, contributing water, food, hydroelectric power, and transport for millennia. Yet an estimated 2.8 million dams now divide up rivers world-wide, threatening healthy river ecosystems and reducing biodiversity in stream systems, in addition to impacts on inland fisheries that supply protein to 158 million people worldwide. Now, the first global assessment of free-flowing rivers has just been published in the journal Nature. The new study finds that as river length increases, so do human impacts. Indeed, only 37% of rivers over 1000 km in length still flow uninterrupted from source to sea, and these are found only in most remote parts of the world: the Arctic, Amazon basin, and the Congo basin. Free flowing rivers are important suppliers of nutrients, sand, and species to deltas and estuaries in particular; and some of the longest remaining (the Irrawaddy and Salween Rivers) produce 1.2 million tons of fish catch annually. Given the global degradation of the natural flow of these long rivers, protection is needed to prevent further declines in their health. The new study was conducted by a global collaboration of 34 scientists from 16 countries led by Günther Grill of McGill University, and included SAFS professor Julian Olden among the coauthors.