Sandra Postel has devoted her life to studying the world's freshwater systems, and they're not looking so great right now. Through a combination of over-allocation, over-engineering, over-use, and climate change, we'll be in trouble if we don't address the problem soon—in fact, we're in trouble now. But the solutions are there, and already in place on a small scale, and they involve working with nature rather than against it to restore the natural flows and stay in balance.