America’s longest river starts at Three Forks Montana
The Missouri is the United States’ longest river, running 2,341 miles (3,767 kilometres) from Montana’s Centennial Mountains to the Mississippi north of St. Louis. Its semi-arid basin exceeds 500,000 square miles—about 529,350 square miles (1,371,000 km²), nearly one-sixth of the country.
Three Rocky streams feed the official start: the Jefferson (via Hell Roaring Creek, Red Rock, and Beaverhead from Brower’s Spring near 9,100 feet on Mount Jefferson), the Madison (Firehole plus Gibbon from Yellowstone), and the Gallatin. They meet in Missouri Headwaters State Park near Three Forks; the Gallatin joins a mile downstream. Farther east the Yellowstone enters as the greatest tributary by volume—larger than the Missouri at the confluence. Near the city of Great Falls it tumbles over five substantial cascades before entering the canyons and badlands of the Missouri Breaks.
Fifteen main-stem dams and hundreds on tributaries remade the twentieth-century basin for irrigation, flood control, and hydropower. Lake Sakakawea, Lake Oahe, and Fort Peck Lake rank as the nation’s largest, second-largest, and fourth-largest artificial lakes by surface area. From Brower’s Spring the river drops 8,626 feet; watershed elevations climb from just over 400 feet at the mouth to Mount Lincoln’s 14,293-foot Colorado summit. Engineers also cut off meanders and straightened the channel for barges, trimming almost 200 miles from its pre-development length.
About twelve million people lived in the basin in 1990, including all of Nebraska and slices of nine other U. S. states plus bits of Alberta and Saskatchewan; Denver was the largest city. More than 170,000 square miles under the plow supply over a third of U. S. wheat, flax, barley, and oats, yet only 11,000 square miles are irrigated. Measured against the Mississippi above their junction, the Missouri is twice as long and drains three times the area, supplying about 45 percent of the flow past St. Louis and up to 70 percent in some droughts. People have leaned on the river for more than 12,000 years, and in the nineteenth century fur trappers, wagon trains and then steamboats made it a principal road west. On its western rim, the Columbia, Missouri and Colorado watersheds meet on one Wyoming peak in the Wind River Range, aptly called Three Waters Mountain.
Source: Missouri River
