By foot, horse, and canoe, European explorers centuries ago undertook years-long expeditions to document the length and breadth of major rivers. Today, satellites make the first pass of discovery. Though rivers meander and melting glaciers birth new lakes annually, the world’s major drainages have largely been mapped. Yet one fundamental dimension remains largely a mystery: the rise and fall of water bodies globally. Accurately measuring, at low-cost, the weekly changes in rivers, lakes, and wetlands would allow scientists to observe how much water moves through them. Land-based gauges do some of this work. But where gauges are scarce — Alaska, Africa, Asian headwaters — these numbers are inaccurate or unknown.