Question: how much of South Dakota is west of the longest river in North America?
Related documents: [0]  [DOC] 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. [END] 
[1]  [DOC] 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. [END] 
Answer: 
over one-half of the land area
over one-half of the land area