Photos showing the aftermath and rescue efforts following the floods in Nepal. A wall of water and mud swept through parts of Nepal at such speed that buildings were engulfed within seconds. As the death toll from the disaster rises, questions are being asked about whether the flooding could have been detected earlier and whether communities downstream could have been given more time to evacuate. Geologists believe the floods in Nepal and Tibet were triggered by a combination of a high-altitude landslide and a glacial avalanche, releasing huge volumes of water. But disaster experts say such events are particularly difficult to forecast. Most early-warning systems for glacial hazards focus on specific, known areas of risk. One common approach is to monitor glacial lakes, which form when retreating glaciers leave depressions that fill with meltwater. Unexpected rises in water levels can provide a warning that a lake could overflow and cause flooding. Eran Hood, a professor of environmental science at the University of Alaska, has studied glacial lake outburst floods from the Mendenhall Glacier and their impact on Juneau, where flooding occurs annually. Scientists there use real-time video, laser measurements of water levels and frequent field observations to assess when flooding might occur and the risks to communities downstream. Experts say the sudden collapse of part of a mountain would have been much harder to anticipate. Detecting such a threat would require intensive monitoring across a vast and remote area. One possible approach would be to monitor seismic activity and use remote-sensing technology to create three-dimensional images of vulnerable areas, Prof Hood told The New York Times. Scientists could then track changes and deformation over time to assess whether an area was at risk of collapsing. "But the Himalayas are very remote. It's not easy to get in there with drones or helicopters," he said. Nepal has 2,070 glacial lakes, according to a 2020 report by an intergovernmental organisation focused on mountain monitoring. Of those, 21 are classified as high risk. There are many more glacial lakes across the wider Himalayan region, including in China and India. Nepal already has a flood warning system, but scientists and disaster-response researchers say its conventional network of river and lake gauges is not designed to detect the sudden collapse of a mountain. The gauges are primarily intended to detect slower increases in water levels associated with monsoon flooding or glacial lake outbursts. They are less suited to the rapidly moving torrents produced by a sudden landslide or glacial collapse. Shushil Kumar Shrestha, a senior divisional engineer at Nepal's National Disaster Risk Reduction and Management Authority, said there was a warning system operating in the affected region. But he said the speed of the flooding left people with almost no time to react. "The flood developed so rapidly that people simply did not have enough time to get to safety, even though the system was functioning," he said.