Nepal could face another flash flood after debris from a suspected ice-rock avalanche accumulated at a river confluence and created a temporary natural dam, an expert from the International Centre for Integrated Mountain Development (ICIMOD) has warned. The temporary blockage is holding back water and could potentially result in another sudden surge if it fails abruptly, adding to the dangers already confronting rescue teams and communities affected by the disaster. Mohd Farooq Azam, a senior cryosphere specialist at ICIMOD, said researchers based near Kathmandu are examining whether an ice-rock avalanche from a high-altitude area triggered the deadly flooding.

The cryosphere refers to portions of the Earth's surface covered by frozen water, including glaciers, snow and ice. Researchers are studying the sequence of events to better understand what caused the catastrophic flooding. “This is basically an ice and rock avalanche,” Azam said, explaining that the current understanding points to a detachment of bedrock beneath a glacier. According to him, material from below the glacier became detached and moved rapidly down into the valley, contributing to the disaster. The avalanche carried a huge volume of rock, ice and other debris into downstream areas. Azam warned that the debris has now accumulated at a river confluence, where it is creating temporary damming.

The resulting blockage is holding back water and forming what can be described as a temporary pond or lake. As more water enters the area, the volume behind the debris could increase substantially over the coming days. According to the ICIMOD specialist, water is expected to continue accumulating in the temporary lake for about three days. This raises the possibility that the natural dam could eventually fail and release a large volume of water downstream. If the release happens suddenly, communities already affected by the initial floods could face another flash flood with little time to respond. The warning comes as Nepal and neighbouring Tibet continue to deal with the devastating consequences of the initial disaster.

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More than 470 people had died across Nepal and Tibet by Friday, while nearly 1,000 people were still reported missing. Search and rescue operations were continuing for a third day as emergency teams worked to locate survivors and recover victims. The possibility of another flood presents an additional challenge for rescue personnel operating in difficult terrain. Teams working near rivers and damaged settlements could themselves be exposed to rapidly changing water levels if the temporary dam gives way. Authorities and emergency agencies therefore face the difficult task of balancing rescue operations with the risk posed by unstable debris and potentially rising water. Natural dams formed by landslide or avalanche debris can behave unpredictably.

Water may accumulate behind the obstruction until it reaches a level at which it begins flowing over or through the debris. If the structure gradually erodes, the water may be released relatively slowly. But if the dam collapses suddenly, the stored water can travel downstream as a powerful flood wave. Azam stressed that a secondary flash flood is only one possible outcome. He said the temporary lake could also breach gradually, allowing the accumulated water to drain without producing another major flood. The way the debris structure responds to increasing water pressure and continued rainfall will therefore be critical in determining the level of risk.

The potential secondary event also highlights the complex chain of natural processes that may have contributed to the disaster. A high-altitude ice-rock avalanche can move enormous quantities of material into valleys, blocking rivers and altering their flow. The resulting changes can create new hazards even after the initial floodwaters have receded. ICIMOD researchers are continuing to assess the suspected avalanche and the temporary blockage as part of efforts to understand the disaster and identify further risks. Monitoring the developing lake will be particularly important over the next several days as water continues to collect behind the debris.

For communities already devastated by the initial flooding, the warning adds another layer of uncertainty. Families are still searching for missing relatives, while rescue teams are recovering bodies and assisting survivors. Any additional flood could further damage infrastructure, complicate access and threaten settlements downstream. The situation therefore remains highly unstable, with authorities and researchers closely watching the temporary lake and surrounding terrain. Whether the stored water drains gradually or the debris dam fails suddenly will determine whether Nepal experiences another major flood in the affected valley. Until the blockage is assessed more fully and the water level stabilises, experts are warning that the danger has not yet passed.

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