largest landslide in history physically changed their world. when the top of that mountain blew off and the north side of that mountain collapsed the debris from that in less than ten minutes filled an adjacent river valley. 25 square miles of that river valley to an average depth of 150 feet in less than ten minutes. imagine taking this huge river valley filling it with degree the height of a 12 to 15-story building, and it all happens in less than ten minutes. one of the rivers that flows through that part of washington, d.c. state is the mighty, mighty columbia river. mount st. helen's eruption basically stopped the columbia river, put sediment and mudslide into the river. to get the columbia river open again. and smaller rivers buried under