CNN  —  The nearly 3 million rivers that weave across the world are experiencing rapid and surprising changes , with potentially drastic implications for everything from drinking water supplies to flood risks, according to a new study. Scientists mapped the flow of water through every single river on the planet, every day over the past 35 years, using a combination of satellite data and computer modeling. What they found shocked them. Nearly half of the world’s largest downstream rivers — 44% —