Global change possesses serious challenges for water managers and scientists. In mountain areas, where water supplies for half of the world population originate, climate and hydrologic models are still subject to considerable uncertainty. And yet, critical decisions have to be taken to ensure adequate and safe water supplies to billions of people, millions of farmers and industries, without further deteriorating rivers and water bodies. While global warming is known to cause glaciers’ retreat and reduced snow packs around the world, it is not clear that mountain discharge will be lower. What is widely recognised is that water management must be adapted to accommodate significant regime changes. However, this inevitably involves managing transboundary rivers, adding further complexity to putting principles in practice.