Question: The Arrow Rock area was where the historic Santa Fe Trail crossed which longest river in North America?
Context: Arrow Rock Historic District is a National Historic Landmark District encompassing the village of Arrow Rock, Missouri and the adjacent Arrow Rock State Historic Site.  The Arrow Rock area was where the historic Santa Fe Trail crossed the Missouri River, and was thus a key stopping point during the settlement of the American West.  The 260 acre historic district was declared a National Historic Landmark in 1963. Arrow Rock State Historic Site is an open-air museum encompassing a geographic formation and a portion of the village of Arrow Rock, Missouri.  The park is part of the Arrow Rock Historic District, a National Historic Landmark, and commemorates the history of the area as a key stop on the Santa Fe Trail. Arrow Rock is a village in Saline County, Missouri, United States, located near the Missouri River.  The village has important historical significance related to westward expansion, the Santa Fe Trail and 19th century artist George Caleb Bingham.  The state’s first state historic site is located here and the entire village is part of the National Historic Landmark Arrow Rock Historic District, designated by the Department of the Interior, National Park Service in 1963.  Many structures within the village are also individually listed on the National Register of Historic Places.  Several locations are also certified sites of the Lewis and Clark National Historic Trail and the Santa Fe National Historic Trail. Santa Fe Trail-Saline County Trail Segments are four historic Santa Fe Trail segments located near Stanhope, Saline County, Missouri.  The four trail rut segments are located on the Weinrich farm.  They date to 1821-1827. Santa Fe Trail-Grand Pass Trail Segments are three historic Santa Fe Trail segments located at Grand Pass, Saline County, Missouri.  The three trail rut segments are located within Maple Hill Cemetery and near Grand Pass Methodist Church.  They date to 1821-1827. 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. Wagon Bed Spring (also Lower Spring or Lower Cimarron Spring), located in Grant County, Kansas, United States, was an important watering spot on the Cimarron Cutoff of the Santa Fe Trail.  The flow of the spring came from an outcropping of the Ogallala Formation.  Center pivot irrigation adjacent to the spring resulted in lowering of the water table and the spring ceased to flow in the 1960s.  A number of small artifacts dating from the days of the Santa Fe Trail have been recovered from lands near the spring which were used by both American Indians and wagon trains as a campground.  The name "wagon bed" dates from later use of an old wagon bed as a trough to collect water from the spring.  There is a foundation on the site of an ice house.  Floods have changed the course of the Cimarron River; the site of the spring is now in the bed of the river rather than on its bank as it was in the days of the Santa Fe Trail. The Great Santa Fe Trail Horse Race Endurance Ride, sometimes referred to as the Santa Fe Ride, is an annual 515-mile equine endurance ride conducted over a fourteen-day period in close proximity to the historic Santa Fe Trail.  The 2007 Ride was won by Scott Griffin of Seattle, Washington, with a combined time of 61 hours 45 minutes over fourteen days.  The 2007 Pioneer Ride was won by Karen Fredrickson of Kneeland, California.  She completed the 515 miles on a named Murphy in a little over 86 hours. Santa Fe Trail Remains, also known as Santa Fe Trail Ruts, is a two-mile (3 km) section of the former 1200 mi long Santa Fe Trail that is the "longest continuous stretch of clearly defined Santa Fe Trail rut remains in Kansas." The Neff Tavern Smokehouse is a historic smokehouse located on the old Santa Fe Trail northeast of Napton, Saline County, Missouri.  It is off Interstate 70 and 6 miles west of Arrow Rock, Missouri.  Missouri pioneer Isaac Neff (aka Isaac Nave) was born in Tennessee in 1797 and died in Missouri in 1878.  He originally built a log tavern on the site in 1837.  The Santa Fe Trail went between the tavern and the barn (later a stage station), skirted the family cemetery, and continued to the northwest.  The tavern was torn down in 1890.  The stone smokehouse is the only remaining original structure on Neff’s former property.
Answer: Missouri River