On Aug. 26, a rock and ice avalanche triggered a huge flood that swept through a steep valley in Nepal, leaving thousands dead and missing. It was an extreme example of a growing problem across the Himalayas. As glaciers melt, glacial lakes expand and mountain permafrost thaws, the risks from floods, landslides and other high-mountain hazards are increasing. At the same time, more people and infrastructure are moving into the valleys where these hazards are most likely to strike.