June 5, 1805 – Scouting ahead of the Corps of Discovery, Meriwether Lewis and four companions sight the Great Falls of the Missouri River.
The Missouri River drops a total of 612 feet from the first of the falls to the last. From upstream to downstream, the five falls, which are located along a 10-mile (16 km) segment of the river, are:
- Black Eagle Falls (26 feet 5 inches)
- Colter Falls (6 feet 7 inches)
- Rainbow Falls (44 feet 6 inches)
- Crooked Falls, also known as Horseshoe Falls (19 feet)
- The Great Falls (87 feet)
Four of the five waterfalls were given names in 1805 by the Corps of Discovery. Both Meriwether Lewis and William Clark named Crooked Falls. Clark named three of the remaining waterfalls on his map: "Great Falls", "Beautiful Cascade", and "Upper Pitch". "Beautiful Cascade" was renamed "Rainbow Falls" in 1872 by Thomas B. Rogers, an engineer with the Great Northern Railway. Colter Falls received its name from Paris Gibson, in honor of John Colter (a member of the Lewis and Clark Expedition). Black Eagle Falls is named for an account in Lewis' journal in which he describes how a black eagle had built a nest in a cottonwood tree on an island in the middle of the falls. It is not clear when the falls lost their original name of "Upper Pitch," but they had acquired their modern name by at least 1877.
The Missouri River 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 Hearne province and Wyoming province. Approximately 1.5 million years ago, the Missouri River, Yellowstone River, and Musselshell River all flowed northward into a terminal lake. During the last glacial period (between 150,000 and 50,000 years ago), the Laurentide and Cordilleran ice sheets pushed these lakes and rivers southward. Between 15,000 and 11,000 BCE, the Laurentide ice sheet blocked the Missouri River and created Glacial Lake Great Falls. The current course of the Missouri River essentially marks the southern boundary of the Laurentide ice sheet. The Missouri, Yellowstone and Musselshell rivers flowed eastward around the glacial mass, eventually settling into their present courses.
The Great Falls themselves formed on a fall line unconformity in the Great Falls Tectonic Zone. The Missouri River settled into a bedrock canyon which lay beneath the clay laid down by Glacial Lake Great Falls. The course of the Missouri in and around the Great Falls has changed very little since then, in comparison to lower regions of the river on the ground moraine that forms much of the upper Great Plains.
The Great Falls of the Missouri River formed because the Missouri is flowing over and through the Kootenai Formation, a mostly nonmarine sandstone laid down by rivers, glaciers, and lakes in the past. Some of the Kootenai Formation is marine, however, laid down by shallow seas. The river is eating away at the softer nonmarine sandstone, with the harder rock forming the falls themselves. Until relatively recently (in geologic time) the Missouri River in the area had a much wider channel, but has now settled into its current course, where it will continue to cut more deeply into the sandstone.
Only one of the waterfalls that comprise the Great Falls of the Missouri River exists in its natural state today. Black Eagle Dam was built in 1890, and half of Black Eagle Falls are now submerged in the Long Pool reservoir behind the dam. Rainbow Dam was built in 1910, and submerged Colter Falls behind it. Volta Dam was built on top of the Great Falls in 1915. (It was renamed Ryan Dam in 1940 in honor of John D. Ryan, the president and founder of the Montana Power Company.)