Humans now capture .50% of available freshwater runoff (Jackson et al. 2001), reservoirs trap 25% of the global sediment load before it reaches the oceans (Vo¨ro¨smarty and Sahagian 2000), and several of the world’s great rivers, including the Ganges– Brahmaputra, Yellow, Nile, and Colorado, have stopped flowing to the sea during dry periods (Postel 2000). River