A river is a mercurial thing, running deep and fast in the rainy season, and low and slow when the rains fade. It can dry up completely one year, then turn into a raging flood the next. Every so often, a river disappears entirely, bringing down the communities it once nourished. You hear a lot about how climate change is fueling the rise of our seas, but not so much about how it will transform our rivers, the flooding of which currently affects almost 60 million people a year.