What caused the flash flood that killed almost 250 people in India’s Uttarakhand state on 7th February 2021? Glacier collapse? Glacial lake outburst? Debris flow? While hypotheses have been plentiful, the underlying evidence is thin. Based on the data now available, we take an objective look at a disaster many consider a portent of global warming.
The first stage in the analysis is to describe the phenomena that occurred in key locations, such as the dam construction site. The videos are harrowing, terrifying. But beyond the horror of the situation, they provide clues to how it happened. They show an extremely powerful flow destroying everything in its path. Although the initial surge is too turbulent and chaotic to make out its composition, once the front has passed, the body of the flow can be seen clearly enough to discern its nature. It is clearly a debris flow—a relatively