This all occurred in a very narrow valley. Picture a deep, high sided V. The glacier collapse must have filled a substantial portion of the V. The river is a high-flow, large volume waterway. The water quickly built up to a large volume.
Meanwhile, the buildings were built close to the bottom of the V in areas the river had, over century’s, dug out. When the glacier plug broke, the raging water picked up boulders and debris and rapidly filled the V shape to a much higher level than it had occupied in the past.
The whole event, from plug to break, probably took a short time. We in America are used to placid, slow moving broad rivers passing through large open plains. It’s hard for us to imagine living somewhere where the only piece of flat land is on display in a museum.
“The water quickly built up to a large volume”
How quickly, though? Hours, days, weeks?