Question: how much of South Dakota is west of the longest river in North America?

full_context: Document 1: Mackenzie River: The Mackenzie River (Slavey language: Deh-Cho [tèh tʃʰò], "big river" or Inuvialuktun: Kuukpak [kuːkpɑk], "great river") is the largest and longest river system in Canada, and is exceeded only by the Mississippi River system in North America. It flows through a vast, isolated region of forest and tundra entirely within the country's Northwest Territories, although its many tributaries reach into four other Canadian provinces and territories. The river's mainstem runs 1738 km in a northerly direction to the Arctic Ocean, draining a vast area nearly the size of Indonesia. It is the largest river flowing into the Arctic from North America, and with its tributaries is one of the longest rivers in the world.
Document 2: Western Australia: Western Australia (abbreviated as WA ) is a state occupying the entire western third of Australia. It is bounded by the Indian Ocean to the north and west, the Great Australian Bight and Southern Ocean to the south, the Northern Territory to the north-east and South Australia to the south-east. Western Australia is Australia's largest state with a total land area of 2,529,875 square kilometres (976,790 sq mi), and the second-largest country subdivision in the world, surpassed only by Russia's Sakha Republic – however, a significant part of it is sparsely populated. The state has about 2.6 million inhabitants, around 11% of the national total. Ninety-two per cent of the population lives in the south-west corner of the state.
Document 3: Columbia River: The Columbia River is the largest river in the Pacific Northwest region of North America. The river rises in the Rocky Mountains of British Columbia, Canada. It flows northwest and then south into the US state of Washington, then turns west to form most of the border between Washington and the state of Oregon before emptying into the Pacific Ocean. The river is 1243 mi long, and its largest tributary is the Snake River. Its drainage basin is roughly the size of France and extends into seven US states and a Canadian province. The fourth-largest river in the United States by volume, the Columbia has the greatest flow of any North American river entering the Pacific.
Document 4: Three Forks, Montana: Three Forks is a city in Gallatin County, Montana, United States and is located within the watershed valley system of both the Missouri and Mississippi rivers drainage basins — and is historically considered the birthplace or start of the Missouri River. The population was 1,869 at the 2010 census. The city of Three Forks is named so because it lies geographically near the point, in nearby Missouri Headwaters State Park, where the Jefferson, Madison, and Gallatin Rivers converge to form the Missouri River — the longest single river in North America, as well as the major portion of the Missouri-Mississippi River System from the headwaters near Three Forks to its discharge into the Gulf of Mexico. Three Forks is part of the Bozeman, MT Micropolitan Statistical Area of approximately 100,000 people and the greater Bozeman demographic area of approximately 125,000 people.
Document 5: Yellowstone River: The Yellowstone River is a tributary of the Missouri River, approximately 692 mi long, in the western United States. Considered the principal tributary of the upper Missouri, the river and its tributaries drain a wide area stretching from the Rocky Mountains in the vicinity of the Yellowstone National Park across the mountains and high plains of southern Montana and northern Wyoming.
Document 6: Iowa: Iowa ( ) is a U.S. state in the Midwestern United States, bordered by the Mississippi River on the east and the Missouri River and the Big Sioux River on the west. Surrounding states include Wisconsin to the northeast, Illinois to the east, Missouri to the south, Nebraska to the west, South Dakota to the northwest, and Minnesota to the north.
Document 7: Marias River: The Marias River is a tributary of the Missouri River, approximately 210 mi (338 km) long, in the U.S. state of Montana. It is formed in Glacier County, in northwestern Montana, by the confluence of the Cut Bank Creek and the Two Medicine River. It flows east, through Lake Elwell, formed by the Tiber Dam, then southeast, receiving the Teton River at Loma, 2 mi. (3.2 km) above its confluence with the Missouri.
Document 8: Mississippi River: The Mississippi River is the chief river of the second-largest drainage system on the North American continent, second only to the Hudson Bay drainage system. Flowing entirely in the United States (although its drainage basin reaches into Canada), it rises in northern Minnesota and meanders slowly southwards for 2320 mi to the Mississippi River Delta at the Gulf of Mexico. With its many tributaries, the Mississippi's watershed drains all or parts of 31 U.S. states and 2 Canadian provinces between the Rocky and Appalachian Mountains. The Mississippi ranks as the fourth-longest and fifteenth-largest river in the world by discharge. The river either borders or passes through the states of Minnesota, Wisconsin, Iowa, Illinois, Missouri, Kentucky, Tennessee, Arkansas, Mississippi, and Louisiana.
Document 9: Missouri: Missouri is a state in the Midwestern United States. With over six million residents, it is the 18th most populous state. The largest urban areas are St. Louis, Kansas City, Springfield, and Columbia. The capitol is in Jefferson City on the Missouri River. The state is the 21st most extensive in area. In the South are the Ozarks, a forested highland, providing timber, minerals, and recreation. The Mississippi River forms the eastern border of the state.
Document 10: Rapid City, South Dakota: Rapid City (Lakota: "Mni Lúzahaŋ Otȟúŋwahe" ; "Swift Water City") is the second-largest city in South Dakota (after Sioux Falls) and the county seat of Pennington County. Named after Rapid Creek, on which the city is established, it is set against the eastern slope of the Black Hills mountain range. The population was 67,956 as of the 2010 Census. Known as the "Gateway to the Black Hills" and the "City of Presidents", it is split by a low mountain ridge that divides the western and eastern parts of the city. Ellsworth Air Force Base is located on the outskirts of the city. Camp Rapid, a part of the South Dakota Army National Guard, is located in the western part of the city. The historic "Old West" town of Deadwood is nearby. In the neighboring Black Hills are the popular tourist attractions of Mount Rushmore, the Crazy Horse Memorial, Custer State Park, and Wind Cave National Park.
Document 11: West River (South Dakota): West River is the portion of the state of South Dakota located west of the Missouri River with well over one-half of the land area and between one-quarter and one-third of the population of the state.
Document 12: Big Sioux River: The Big Sioux River is a tributary of the Missouri River, 419 miles (674 km) long, in eastern South Dakota and northwestern Iowa in the United States. The United States Board on Geographic Names settled on "Big Sioux River" as the stream's name in 1931. The river was named after the Lakota people (Sioux Indians).
Document 13: Missouri River: 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 2341 mi 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 km), 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.
Document 14: Red Rock River (Montana): The Red Rock River is a roughly 70 mi river in southwestern Montana in the United States. Its drainage basin covers over 1548 mi2 . Its furthest tributary, Hell Roaring Creek, originates in the Beaverhead National Forest within a few hundred meters of the North American Continental Divide and Montana-Idaho border near Brower's Spring, at an elevation of about 9100 ft . Brower's Spring is near the furthest headwaters of the Missouri River, one of the major watercourses of the central United States. The drainage flows north and west with its name changing to "Red Rock Creek" into the Red Rock Lakes in the middle of a wide grassy valley; the Red Rock River issues from the west side of Lower Red Rock Lake. It flows west, receiving many tributaries such as Peet Creek and Long Creek, widening into the Lima Reservoir and then passing through a canyon, which ends near Lima, Montana. From there, it flows northwest through a valley, passing Kidd and Red Rock, and into Clark Canyon Reservoir. Under the waters of the lake was once the confluence of the Red Rock and Horse Prairie Creek, forming the Beaverhead River, a tributary of the Jefferson River, in turn a headwater of the Missouri River.
Document 15: Missouri River Valley: The Missouri River Valley outlines the journey of the Missouri River from its headwaters where the Madison, Jefferson and Gallatin Rivers flow together in Montana to its confluence with the Mississippi River in the State of Missouri. At 2300 mi long the valley drains one-sixth of the United States, and is the longest river valley on the North American continent. The valley in the Missouri River basin includes river bottoms and floodplains.
ground_truth: over one-half of the land area