AP Tolang / ShutterstockOn August 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. Our new study shows how quickly this exposure is growing. We focused on Nepal, and the catchment areas of rivers that drain into Nepal. In that region, between 2016 and 2023, the built-up areas potentially exposed to flooding increased by 46%. Across the Nepalese Himalayas, we estimate almost 300,000 people live in these flood-prone areas, including more than 100,000 downstream of glacial lakes.Those lakes, formed in depressions carved out by retreating glaciers, are a particular risk because they can suddenly breach, sending flood waters crashing down steep valleys. So far, scientists have largely focused on what’s causing these hazards: the melting glaciers, landslides, torrential monsoon rains and so on. Far less attention has been paid to exposure: the people, buildings, and infrastructure potentially at risk. Our results reveal a concerning trend of continued urbanisation in areas susceptible to flooding. This is partly due to the geography. In the Himalayas, valley floors are some of the only flat and accessible places to build roads, settlements and other infrastructure. Rivers also provide natural routes through the mountains – the recent disaster struck Nepal’s primary trade route to China running along the Bhotekoshi-Trishuli rivers, for instance. Humans are expanding further into the Himalayas. Rei Imagine / shutterstock Yet as trade and tourism grow, these corridors attract new roads, tracks, lodges, hydropower projects and more. And once a road connects a valley, other development often follows, including homes, schools and healthcare facilities. But these same valleys are also the natural pathways for floods, debris flows and landslides.Measuring flood exposureWe measured exposure to potential flooding using a metric called “height above nearest drainage”, or Hand, which estimates how high a building sits above the nearest river channel. In steep mountain valleys, two locations may be only a short distance apart horizontally, but separated by tens of metres vertically, resulting in very different flood exposure. In a flood like this one in Kathmandu Valley in 2024, some parts of town can be entirely destroyed, while slightly higher streets and buildings are left untouched. Pratisha Manandhar / shutterstock This makes it particularly difficult to produce flood hazard models in steep Himalayan valleys, which often lack detailed topographic data. Height above nearest drainage does not replace the critical need for this detailed flood hazard mapping, but offers a simple screening metric to identify exposure hotspots. We combined it with satellite-derived building data to track changes in flood exposure between 2016 and 2023.Preparing for the futureOur study does not suggest that floods are inevitable everywhere or that all development near rivers should stop. Exposure to the flood waters themselves is only one component of risk, and other factors such as building type, early warning systems, preparedness and evacuation planning all influence whether a hazardous event becomes a disaster. Nonetheless, our findings reveal a clear trend, that exposure to potential flooding is rising and that development decisions being made today will shape disaster risk for the coming decades.Climate change is making some Himalayan hazards more dangerous. But disaster risk is also being shaped by where people live and work. Identifying the places where exposure is already high, and incorporating that information into land and infrastructure planning, could help prevent today’s new developments turning into disaster zones of the future.C. Scott Watson receives funding from a UKRI Future Leaders Fellowship (grant number MR/Y016564/1).Milan Shrestha received funding from the National Science Foundation of the USA (CNH Grant # 1516912 between 2015-2019) to study a research project "Science-Driven, Community-Based Approach to Reducing Glacier Lake Outburst Flood Risks in Nepal"