Four hundred million people--if it were a country, it would be the third largest in the world--rely on the Ganges River and its tributaries for their livelihood. Six thousand rivers provide a perennial source of irrigation and power to one of the world’s most densely populated and poorest areas. The Himalayas, “the water tower of the Ganges,” provide 45 percent of the annual flow. These facts represent the potential payoffs to the populations of Bangladesh, India and Nepal as well as the threat that climate change poses to poor and already <