If you look at the two mountains east of Missoula (Mt Jumbo and Mt Sentinel) you may notice what seems to be horizontal lines on the mountains. About 12,000 years ago, the valleys of western Montana lay beneath a lake nearly 2,000 feet deep, referred to as Glacial Lake Missoula. The lake was formed as an ice sheet damned the Clark Fork River just as it entered Idaho. Eventually, the rising water behind the dam weakened it until water burst through in a catastrophic flood that raced to the Pacific Ocean. The waves and ice tore away soils and mountainsides, creating the scablands of eastern Washington and carved the Columbia River Gorge. Over the course of centuries, Glacial Lake Missoula filled and emptied in repeated cycles, leaving its story embedded in the land and the sides of the mountains around Missoula. The lake was as big as Lakes Erie and Ontario combined, and the flood waters ran with the force equal to 60 Amazon Rivers. Car-sized boulders embedded in ice floated some 500 miles; they can still be seen today!