where does missouri river end?
Mississippi River
entailment
 The Missouri River is the longest river in North America . Rising in the Rocky Mountains of western Montana , the Missouri flows east and south for 2 @,@ 341 miles ( 3 @,@ 767 km ) before entering the Mississippi River north of St. Louis , Missouri . The river takes drainage from a sparsely populated , semi @-@ arid watershed of more than half a million square miles ( 1 @,@ 300 @,@ 000 km2 ) , which includes parts of ten U.S. states and two Canadian provinces . When combined with the lower Mississippi River , it forms the world 's fourth longest river system . 

entailment
 The Missouri and many of its tributaries cross the Great Plains , flowing over or cutting into the Ogallala Group and older mid @-@ Cenozoic sedimentary rocks . The lowest major Cenozoic unit , the White River Formation , was deposited between roughly 35 and 29 million years ago and consists of claystone , sandstone , limestone , and conglomerate . Channel sandstones and finer @-@ grained overbank deposits of the fluvial Arikaree Group were deposited between 29 and 19 million years ago . The Miocene @-@ age Ogallala and the slightly younger Pliocene @-@ age Broadwater Formation deposited atop the Arikaree Group , and are formed from material eroded off of the Rocky Mountains during a time of increased generation of topographic relief ; these formations stretch from the Rocky Mountains nearly to the Iowa border and give the Great Plains much of their gentle but persistent eastward tilt , and also constitute a major aquifer . 

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 The Missouri River officially starts at the confluence of the Jefferson and Madison in Missouri Headwaters State Park near Three Forks , Montana , and is joined by the Gallatin a mile ( 1 @.@ 6 km ) downstream . The Missouri then passes through Canyon Ferry Lake , a reservoir west of the Big Belt Mountains . Issuing from the mountains near Cascade , the river flows northeast to the city of Great Falls , where it drops over the Great Falls of the Missouri , a series of five substantial waterfalls . It then winds east through a scenic region of canyons and badlands known as the Missouri Breaks , receiving the Marias River from the west then widening into the Fort Peck Lake reservoir a few miles above the confluence with the Musselshell River . Farther on , the river passes through the Fort Peck Dam , and immediately downstream , the Milk River joins from the north . 

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 The Missouri 's headwaters above Three Forks extend much farther upstream than the main stem . Measured to the farthest source at Brower 's Spring , the Jefferson River is 298 miles ( 480 km ) long . Thus measured to its highest headwaters , the Missouri River stretches for 2 @,@ 639 miles ( 4 @,@ 247 km ) . When combined with the lower Mississippi , the Missouri and its headwaters form part of the fourth @-@ longest river system in the world , at 3 @,@ 745 miles ( 6 @,@ 027 km ) . 

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 With 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 and drains an area three times as large . 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 . 
