ith a drainage basin spanning 529,350 square miles (1,371,000 km2), the Missouri River's catchment encompasses nearly one-sixth of the area of the United States or just over five percent of the continent of North America. Comparable to the size of the Canadian province of Quebec, the watershed encompasses most of the central Great Plains, stretching from the Rocky Mountains in the west to the Mississippi River Valley in the east and from the southern extreme of western Canada to the border of the Arkansas River watershed. Compared with the Mississippi River above their confluence, the Missouri is twice as long[n 2] and drains an area three times as large.[n 3] The Missouri accounts for 45 percent of the annual flow of the Mississippi past St. Louis, and as much as 70 percent in certain droughts. In 1990, the Missouri River watershed was home to about 12 million people. This included the entire population of the U.S. state of Nebraska, parts of the U.S. states of Colorado, Iowa, Kansas, Minnesota, Missouri, Montana, North Dakota, South Dakota, and Wyoming, and small southern portions of the Canadian provinces of Alberta and Saskatchewan. The watershed's largest city is Denver, Colorado, with a population of more than six hundred thousand. Denver is the main city of the Front Range Urban Corridor whose cities had a combined population of over four million in 2005, making it the largest metropolitan area in the Missouri River basin. Other major population centers – mostly in the watershed's southeastern portion – include Omaha, Nebraska, north of the confluence of the Missouri and Platte Rivers; Kansas City, Missouri – Kansas City, Kansas, at the confluence of the Missouri with the Kansas River; and the St. Louis metropolitan area, south of the Missouri River just below the latter's mouth, on the Mississippi. In contrast, the northwestern part of the watershed is sparsely populated. However, many northwestern cities, such as Billings, Montana, are among the fastest growing in the Missouri basin. With more than 170,000 square miles (440,000 km2) under the plow, the Missouri River watershed includes roughly one-fourth of all the agricultural land in the United States, providing more than a third of the country's wheat, flax, barley, and oats. However, only 11,000 square miles (28,000 km2) of farmland in the basin is irrigated. A further 281,000 square miles (730,000 km2) of the basin is devoted to the raising of livestock, mainly cattle. Forested areas of the watershed, mostly second-growth, total about 43,700 square miles (113,000 km2). Urban areas, on the other hand, comprise less than 13,000 square miles (34,000 km2) of land. Most built-up areas are along the main stem and a few major tributaries, including the Platte and Yellowstone Rivers. The sun low over the horizon over a body of water surrounded by dark vegetation The Missouri in North Dakota, which was the furthest upstream that French explorers traveled on the river Elevations in the watershed vary widely, ranging from just over 400 feet (120 m) at the Missouri's mouth to the 14,293-foot (4,357 m) summit of Mount Lincoln in central Colorado. The river drops 8,626 feet (2,629 m) from Brower's Spring, the farthest source. Although the plains of the watershed have extremely little local vertical relief, the land rises about 10 feet per mile (1.9 m/km) from east to west. The elevation is less than 500 feet (150 m) at the eastern bord