Glacial Lake Missoula was a prehistoric proglacial lake

Proglacial lake

In geology, a proglacial lake is a lake formed either by the damming action of a moraine or ice dam during the retreat of a melting glacier, or by meltwater trapped against an ice sheet due to isostatic depression of the crust around the ice...

 in western Montana

Montana

Montana is a state in the Western United States. The western third of Montana contains numerous mountain ranges. Smaller, "island ranges" are found in the central third of the state, for a total of 77 named ranges of the Rocky Mountains. This geographical fact is reflected in the state's name,...

 that existed periodically at the end of the last ice age

Ice age

An ice age or, more precisely, glacial age, is a generic geological period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in the presence or expansion of continental ice sheets, polar ice sheets and alpine glaciers...

 between 15,000 and 13,000 years ago. The lake measured about 7770 square kilometres (3,000 sq mi) and contained about 2100 cubic kilometres (503.8 cu mi) of water, half the volume of Lake Michigan

Lake Michigan

Lake Michigan is one of the five Great Lakes of North America and the only one located entirely within the United States. It is the second largest of the Great Lakes by volume and the third largest by surface area, after Lake Superior and Lake Huron...

.

The Glacial Lake Missoula National Natural Landmark is located about 68 miles northwest of Missoula, Montana

Missoula, Montana

Missoula is a city located in western Montana and is the county seat of Missoula County. The 2010 Census put the population of Missoula at 66,788 and the population of Missoula County at 109,299. Missoula is the principal city of the Missoula Metropolitan Area...

 at the north end of the Camas Prairie Valley, just east of Montana Highway 382 and Macfarlane Ranch.