Most people in this region live in river valleys. A river valley is a low land between mountains. It has a river flowing through it. The flood started when a glacier collapsed. This made a landslide. A landslide is a sudden movement of soil or rock from higher ground to lower ground. The landslide was powerful. At first scientists believed it was an earthquake. It caused a giant wave of ice, water, mud, and rocks when it hit the river.
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