The Missouri and Yellowstone Rivers look on the map like a big crab’s pincer, joined to the leg just over the North Dakota border and grasping a big chunk of eastern Montana. Western rivers such as the Clark Fork of the Columbia, the Flathead, and the Kootenai are tucked into mountain valleys. Rivers provided the way into Montana for the first white explorers, who were hoping to sail out on the Columbia. Trappers and traders also used the rivers as thoroughfares. Steamboats made the difficult trip up the Missouri from St. Louis through sandbars, rapids, and fallen trees to Fort Benton—until railroads took over the transportation business. Towns sprang up in the river valleys, and highways were built on their banks.

The dams that now harness water power and create reservoirs across the state have altered Montana’s geography. Every large river but the Yellowstone has been dammed at some point, and millions of gallons of water are backed up in Fort Peck Reservoir on the Missouri and in Lake Koocanusa on the Kootenai River. The Great Falls of the Missouri, which took Lewis and Clark 22 backbreaking days to portage in 1805, are now a series of hydroelectric dams.