Missouri River in the context of "Oregon Trail"

St. Louis (/seɪnt ˈluːɪs, sənt-/ saynt LOO-iss, sənt-, sometimes referred to as St. Louis City, Saint Louis or STL) is an independent city in the U.S. state of Missouri. It lies near the confluence of the Mississippi and the Missouri rivers. In 2020, the city proper had a population of 301,578, while its metropolitan area, which extends into Illinois, had an estimated population of over 2.8 million. It is the largest metropolitan area in Missouri and the second-largest in Illinois. The city's combined statistical area is the 20th-largest in the United States.

The land that became St. Louis had been occupied by Native American cultures for thousands of years before European settlement. The city was founded on February 14, 1764, by French fur traders Gilbert Antoine de St. Maxent, Pierre Laclède, and Auguste Chouteau. They named it for King Louis IX of France, and it quickly became the regional center of the French Illinois Country. In 1804, the United States acquired St. Louis as part of the Louisiana Purchase. In the 19th century, St. Louis developed as a major port on the Mississippi River; from 1870 until the 1920 census, it was the fourth-largest city in the country. It separated from St. Louis County in 1877, becoming an independent city and limiting its political boundaries. In 1904, it hosted the Louisiana Purchase Exposition, also known as the St. Louis World's Fair, and the Summer Olympics.

The Mississippi River System, also referred to as the Western Rivers, is a mostly riverine network of the United States which includes the Mississippi River and connecting waterways. The Mississippi River is the largest drainage basin in the United States. In the United States, the Mississippi drains about 41% of the country's rivers.

From the perspective of natural geography and hydrology, the system consists of the Mississippi River itself and its numerous natural tributaries and distributaries. The major tributaries are the Arkansas, Illinois, Missouri, Ohio and Red rivers. Given their flow volumes, major Ohio River tributaries like the Allegheny, Tennessee, and Wabash rivers are considered important tributaries to the Mississippi system. Before the Mississippi River reaches the Gulf of Mexico, it runs into its distributary, the Atchafalaya River.

The Mississippi River is the primary river of the largest drainage basin in the United States. It is the second-longest river in the United States, behind only the Missouri. From its traditional source of Lake Itasca in northern Minnesota, it flows generally south for 2,340 mi (3,770 km) to the Mississippi River Delta in the Gulf of Mexico. With its many tributaries, the Mississippi's watershed drains all or parts of 32 U.S. states and two Canadian provinces between the Rocky and Appalachian mountains. The river either borders or passes through the states of Minnesota, Wisconsin, Iowa, Illinois, Missouri, Kentucky, Tennessee, Arkansas, Mississippi, and Louisiana. The main stem is entirely within the United States; the total drainage basin is 1,151,000 sq mi (2,980,000 km), of which only about one percent is in Canada. The Mississippi ranks as the world's tenth-largest river by discharge flow, and the largest in North America.

Native Americans have lived along the Mississippi River and its tributaries for thousands of years. Many were hunter-gatherers, but some, such as the Mound Builders, formed prolific agricultural and urban civilizations, and some practiced aquaculture. The arrival of Europeans in the 16th century changed the native way of life as first explorers, then settlers, ventured into the basin in increasing numbers. The river served sometimes as a barrier, forming borders for New Spain, New France, and the early United States, and throughout as a vital transportation artery and communications link. In the 19th century, during the height of the ideology of manifest destiny, the Mississippi and several tributaries, most notably its largest, the Ohio and Missouri, formed pathways for the western expansion of the United States. The river also became the subject of American literature, particularly in the writings of Mark Twain.

Missouri (see pronunciation) is a landlocked state in the Midwestern region of the United States. Ranking 21st in land area, it borders Iowa to the north, Illinois, Kentucky and Tennessee to the east, Arkansas to the south and Oklahoma, Kansas, and Nebraska to the west. In the south are the Ozarks, a forested highland, providing timber, minerals, and recreation. At 1.5 billion years old, the St. Francois Mountains are among the oldest in the world. The Missouri River, after which the state is named, flows through the center and into the Mississippi River, which forms the eastern border. With over six million residents, it is the 19th-most populous state of the country. The largest urban areas are St. Louis, Kansas City, Springfield, and Columbia. The capital is Jefferson City.

Humans have inhabited present-day Missouri for at least 12,000 years. The Mississippian culture, which emerged in the ninth century, built cities with pyramidal and other ceremonial mounds before declining in the 14th century. The Indigenous Osage and Missouria nations inhabited the area when European people arrived in the 17th century. The French incorporated the territory into Louisiana, founding Ste. Genevieve in 1735 and St. Louis in 1764. After a brief period of Spanish rule, the United States acquired Missouri as part of the Louisiana Purchase in 1803. Americans from the Upland South rushed into the new Missouri Territory, taking advantage of its productive agricultural plains; Missouri played a central role in the westward expansion of the United States. Missouri was admitted as a slave state as part of the Missouri Compromise of 1820. As a border state, Missouri's role in the American Civil War was complex, and it was subject to rival governments, raids, and guerilla warfare. After the war, both Greater St. Louis and the Kansas City metropolitan area became large centers of industrialization and business.

The Illinois Country (French: Pays des Illinois [pɛ.i dez‿i.li.nwa]; lit. 'land of the Illinois people'; Spanish: País de los ilinueses), also referred to as Upper Louisiana (French: Haute-Louisiane [ot.lwi.zjan]; Spanish: Alta Luisiana), was a vast region of New France claimed in the 1600s that later fell under Spanish and British control before becoming what is now part of the Midwestern United States. While the area claimed included the entire Upper Mississippi River watershed, French colonial settlement was concentrated along the Mississippi and Illinois Rivers in what is now the U.S. states of Illinois and Missouri, with outposts on the Wabash River in Indiana. Explored in 1673 from Green Bay to the Arkansas River by the Canadien expedition of Louis Jolliet and Jacques Marquette, the area was claimed by France. It was settled primarily from the Pays d'en Haut in the context of the fur trade, and in the establishment of missions from Canada by French Catholic religious orders. Over time, the fur trade took some French to the far reaches of the Rocky Mountains, especially along the branches of the broad Missouri River valley. The "Illinois" in the territory's name is a reference to the Illinois Confederation, a group of related Algonquian native peoples.

The Illinois Country was governed from the French province of Canada until 1717 when, by order of King Louis XV, it was annexed to the French province of Louisiana, becoming known as "Upper Louisiana". By the mid-18th century, major settlements included Cahokia, Kaskaskia, Chartres, Saint Philippe, and Prairie du Rocher, all on the east side of the Mississippi in present-day Illinois; and Ste. Genevieve across the river in what is now Missouri, as well as Fort Vincennes in what is now Indiana.

St. Louis County is located in eastern Missouri. It is bounded by the City of St. Louis and the Mississippi River to the east, the Missouri River to the north, and the Meramec River to the south. As of the 2020 census, the population was 1,004,125, making it the most populous county in Missouri, and was estimated to be 987,059 in 2023. Its county seat is Clayton. The county is included in the St. Louis, MO–IL metropolitan statistical area.

After Great Britain took over former French territory east of the Mississippi River, many ethnic French colonists moved west. They settled the area of St. Louis County and founded the city of St. Louis in the late 1700s. The US acquired this territory in 1803 with the Louisiana Purchase.

The Upper Mississippi River is the portion of the Mississippi River upstream of St. Louis, Missouri, United States, a city at the confluence of its main tributary, the Missouri River. Historically, it may refer to the area above the Arkansas Post, above the confluence of Ohio River, or above Cape Girardeau.

The Arkansas River is a major tributary of the Mississippi River. It generally flows to the east and southeast as it traverses the U.S. states of Colorado, Kansas, Oklahoma, and Arkansas. The river's source basin lies in Colorado, specifically the Arkansas River Valley. The headwaters derive from the snowpack in the Sawatch and Mosquito mountain ranges. It flows east into Kansas and finally through Oklahoma and Arkansas, where it meets the Mississippi River.

At 1,469 miles (2,364 km), it is the sixth-longest river in the United States, the second-longest tributary in the Mississippi–Missouri system, and the 47th longest river in the world. Its origin is in the Rocky Mountains in Lake County, Colorado, near Leadville. In 1859, placer gold discovered in the Leadville area brought thousands seeking to strike it rich, but the easily recovered placer gold was quickly exhausted. The Arkansas River's mouth is at Napoleon, Arkansas, and its drainage basin covers nearly 170,000 square miles (440,000 km). Its volume is much smaller than the Missouri and Ohio rivers, with a mean discharge of about 40,000 cubic feet per second (1,100 m/s).