<P> The Missouri River was one of the main routes for the westward expansion of the United States during the 19th century . The growth of the fur trade in the early 19th century laid much of the groundwork as trappers explored the region and blazed trails . Pioneers headed west en masse beginning in the 1830s, first by covered wagon, then by the growing numbers of steamboats entering service on the river . Former Native American lands in the watershed were taken over by settlers, leading to some of the most longstanding and violent wars against indigenous peoples in American history . </P> <P> During the 20th century, the Missouri River basin was extensively developed for irrigation, flood control and the generation of hydroelectric power . Fifteen dams impound the main stem of the river, with hundreds more on tributaries . Meanders have been cut and the river channelized to improve navigation, reducing its length by almost 200 miles (320 km) from pre-development times . Although the lower Missouri valley is now a populous and highly productive agricultural and industrial region, heavy development has taken its toll on wildlife and fish populations as well as water quality . </P> <P> From the Rocky Mountains of Montana and Wyoming, three streams rise to form the headwaters of the Missouri River: v </P> <Ul> <Li> the longest begins near Brower's Spring, 9,100 feet (2,800 m) above sea level on the southeastern slopes of Mount Jefferson in the Centennial Mountains . From there it flows west then north; runs first in Hell Roaring Creek then west into the Red Rock; swings northeast to become the Beaverhead River; and finally joins with the Big Hole to form the Jefferson River </Li> <Li> the Firehole River, originating at Yellowstone National Park's Madison Lake, joins with the Gibbon to form the Madison River, and </Li> <Li> rising out of Gallatin Lake, also in the national park, flows the Gallatin River </Li> </Ul>

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