Missouri, river, c.2,565 mi (4,130 km) long (including its Jefferson-Beaverhead-Red Rock headstream), the longest river of the United States and the principal tributary of the Mississippi River. The length of the combined Missouri-Mississippi system from the headwaters of the Missouri to the mouth of the Mississippi is c.3,740 mi (6,020 km), making it the world's third longest river after the Nile and the Amazon. The Missouri River drains an area of c.580,000 sq mi (1,502,200 sq km), including 2,550 sq mi (6,600 sq km) in Canada.
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