Great Falls, Montana - Geography and Climate

Geography and Climate

Great Falls is located at 47°30′13″N 111°17′11″W / 47.50361°N 111.28639°W / 47.50361; -111.28639 (47.503657, -111.286299), near several waterfalls on the Missouri River. It lies near the center of Montana on the northern Great Plains. It lies next to the Montana Rocky Mountain Front and is about 100 miles (160 km) south of the Canadian border.

The city of Great Falls lies atop the Great Falls Tectonic Zone, an intracontinental shear zone between two geologic provinces of basement rock of the Archean period which form part of the North American continent. The city lies at the southern reach of the Laurentide ice sheet, a vast glacial sheet of ice which covered much of North America during the last glacial period. Approximately 1.5 million years ago, the Missouri River flowed northward into a terminal lake. The Laurentide ice sheet pushed the river southward. Between 15,000 BCE and 11,000 BCE, the Laurentide ice sheet blocked the Missouri River and created Glacial Lake Great Falls. About 13,000 BCE, as the glacier retreated, Glacial Lake Great Falls emptied catastrophically in a glacial lake outburst flood. The current course of the Missouri River essentially marks the southern boundary of the Laurentide ice sheet. The Missouri River flowed eastward around the glacial mass, settling into its present course. As the ice retreated, meltwater from Glacial Lake Great Falls poured through Highwood Mountains and eroded the mile-long, 500-foot-deep (150 m) Shonkin Sag—one of the most famous prehistoric meltwater channels in the world.

Great Falls is also situated on a fall line unconformity in the Great Falls Tectonic Zone, as well as atop the Kootenai Formation, a mostly nonmarine sandstone laid down by rivers, glaciers, and lakes in the past.

According to the United States Census Bureau, the city has a total area of 19.9 square miles (52 km2), of which, 19.5 square miles (51 km2) of it is land and 0.4 square miles (1.0 km2) of it (2.21%) is water.

Great Falls has a semi-arid climate (Köppen climate classification BSk), with a larger part of precipitation occurring in the form of thunderstorms. Winters are cold and somewhat snowy, though chinook winds help to greatly moderate them. In the absence of such winds, shallow cold snaps extending well below 0 °F (−18 °C) are common. Summers are warm and somewhat dry, with highs reaching 90 °F (32 °C) on 18 days per year, though temperatures at nights are sharply cooler than daytime conditions. Freak snowfalls such as a one-day total of 6.6 inches (16.8 cm) in August 1992 can occur.