Today 's Missoula lies at the bottom of what once was Glacial Lake Missoula , a proglacial lake which stretched from south and east of Missoula north to today 's Flathead Lake and west to Idaho 's Lake Pend Oreille . Held in place by a glacial dam , this lake drained and refilled repeatedly over 2,000 years during the past Ice Age . When the flood waters cleared , the resultant Missoula Valley became a geographic hub of five mountain valleys formed by the Bitterroot Mountains , Sapphire Range , Garnet Range , Rattlesnake Mountains , and Reservation Divide
drained and refilled repeatedly
Missoula Floods
History of Missoula, Montana