The American Bottom is the flood plain of the Mississippi River in the Metro-East region of Southern Illinois, extending from Alton, Illinois, south to the Kaskaskia River. It is also sometimes called "American Bottoms". The area is about 175 square miles (450 km2), mostly protected from flooding in the 21st century by a levee and drainage canal system. Immediately across the river from St. Louis, Missouri, are industrial and urban areas, but nearby marshland, swamps, and the Horseshoe Lake (which was created by the river) are reminders of the Bottoms' riparian nature.
This plain with its rich alluvial soil, served as the center for the pre-Columbian Cahokia Mounds civilization, and later the French settlement of Illinois Country. Deforestation of the river banks in the 19th century to fuel steamboats had dramatic environmental effects in this region. The Mississippi River between St. Louis and the confluence with the Ohio River became wider and more shallow, as unstable banks collapsed into the water.  This resulted in more severe flooding and lateral changes of the major channel, causing the destruction of several French colonial towns, such as Kaskaskia, which relocated; Cahokia, and St. Philippe, Illinois.
The southern portion of the American Bottoms is primarily agricultural, planted chiefly in corn, wheat, and soybean. The American Bottom is part of the Mississippi Flyway used by migrating birds and has the greatest concentration of bird species in Illinois. The flood plain is bounded on the east by a nearly continuous, 200- to 300-foot high, 80-mile (130 km) long bluff of limestone and dolomite, above which begins the great prairie that covers most of the state. The Mississippi River bounds the Bottom on its west, and the river abuts the bluffline on the Missouri side. Portions of St. Clair, Madison, Monroe, and Randolph counties are in the American Bottom. Its maximum width is about 9 miles (14 km) in the north, and it is about 2 to 3 miles in width throughout most of its southern extent.
Before European settlement, the area was home to indigenous peoples for many centuries. The peak civilization was created by peoples of the Mississippian culture, known as the Mound Builders. With the cultivation of maize, they were able to create food surpluses and build concentrated settlements in the centuries after 600 CE. The Cahokia Mounds Site, which was built as the center attracted a rapid increase in population after 1000 CE, is a six-square mile complex of large, man-made, earthen mounds rising from the flood plain. In 1982, it was designated by UNESCO as one of only eight World Heritage Sites in the United States.
The most prominent structure is Monks Mound, rising ten stories high at the center of the complex and fronting on a 40-acre (160,000 m2) Grand Plaza. Monks Mound is the largest Pre-Columbian earthwork in the Americas, and the complex is the largest earthwork north of Mexico. The engineering of the mounds showed that their builders had an expert knowledge of the varying soils and their capacities. Cahokia was a complex, planned, and designed urban center with a residential population, farming, and artisan production of refined crafts and goods. With its location at the confluence of three major rivers, it was the center of a regional trading network reaching to the Great Lakes and the Gulf Coast. With a population estimated at 30,000 at its peak, Cahokia was the largest city north of modern-day Mexico. Perhaps for ecological reasons—deforestation and overhunting—the city went into decline after 1300 and was abandoned before 1400. No city in the territorial United States surpassed this population until after 1800, when Philadelphia exceeded it.
Archaeological investigation has determined that the various types of mounds were arranged in a planned construction that reflected the cosmology of the Mississippians. The smaller ridge-top and conical mounds were used for ritual burials, some for elites and some for apparent sacrifices. The larger platform mounds were used for temples and homes of the elite. Archaeologists have found remains of a 2-mile (3.2 km) long, defensive wooden stockade that enclosed the central precinct and was rebuilt several times. They also discovered two major solar calendars, now known as Woodhenge, as the works were constructed of cedar, which was considered a sacred wood. The area surrounding the mounds had numerous borrow pits from which soil was taken to build the mounds and to fill and level the Grand Plaza and other plazas.
After Cahokia was abandoned, there were few indigenous inhabitants in the area in the 17th century at the time of first French exploration.  The French made the earliest European settlement in this region of the Mississippi River Valley. They encountered Illiniwek clans called Cahokia, after whom they named the earthwork complex, and Kaskaskia, after whom the French named a river, a town and a fort. The French villages included Kaskaskia, Cahokia, Prairie du Rocher, St. Philippe, and Prairie du Pont, all under the protection of Fort de Chartres and Fort Kaskaskia. Examples of 18th-century French Colonial architecture survive here, including the old Cahokia courthouse and Holy Family Catholic Church, both made with the distinctive vertical-log construction known as poteaux-sur-solle .
American settlers began arriving near the end of the American Revolution after the Illinois Country was ceded by Great Britain to the new United States. In the early years, American single men came to the country, and there was little government and much anarchy. As Americans arrived, many residents of French descent moved west of the Mississippi River to St. Louis and Ste. Genevieve, Missouri. Within several years, the former French colonial towns had become mostly American in population, and English dominated as the language.
The Goshen Settlement was an early American settlement at the edge of the Bottom. The settlers continued to use the rich alluvial floodplain mostly for agriculture until the late 19th century. Brooklyn, Illinois was founded by 1839 as a freedom village by free people of color and fugitive slaves, led by "Mother" Priscilla Baltimore. It was the first town incorporated by African Americans under a state legal system. 
The rivers were used as transportation routes for trading and travel. The introduction of steamboats to the Mississippi and other major rivers led to deforestation of the river banks in the 19th century. The steamboats consumed much wood for fuel, leading to dramatic environmental effects along the Mississippi River between St. Louis and the confluence with the Ohio River. With so many trees taken down, the banks became unstable, collapsing into the river due to the powerful current. In this area, the Mississippi became wider and more shallow, which resulted in more severe flooding and lateral changes of the major channel. Several French colonial towns in the 19th century, such as Kaskaskia, Cahokia, and St. Philippe, Illinois, were flooded and destroyed.  Kaskaskia was rebuilt but, after the channel of the Mississippi shifted, it was cut off from the mainland of Illinois and is connected to Missouri land.
The area of the Bottom directly across from St. Louis became highly industrialized. Industrialists located many "smokestack" industries here, such as steel mills, chemical plants, and oil refineries, because they ran on Illinois coal. In addition, the people who built the first bridge from St. Louis across the Mississippi River to Illinois imposed a tax on heavy traffic. Rather than pay it, developers simply located their industries in East St. Louis.
In the early 20th century, dramatic growth in industrial jobs in the American Bottom attracted many European immigrants and African-American migrants. The latter left the rural South in the Great Migration to work in factories and gain better lives for their children. Eastern European immigrants founded the first Bulgarian Orthodox Church in the United States in Madison, Illinois. Today, East St. Louis is predominantly African-American in ethnicity. Following industrial restructuring, other immigrant descendants moved to other areas when following jobs and housing. Heavy industry is still prominent in the area, although total employment in these industries continues to decline after restructuring and industry changes.
Like the Mississippians, Americans made massive changes in the floodplain; their development has reduced its ability to absorb floods. The destruction of wetlands and paving over of areas along all the major rivers has increased the severity of flooding over the decades, despite attempted engineering solutions for flood control, which in turn have exacerbated flooding. During the Great Flood of 1993, major portions of the southern Bottom were flooded; 47,000 acres (190 km²) of land below Columbia, Illinois was inundated, destroying the town of Valmeyer. The waters came within five feet of overtopping the East St. Louis levee. If they had run over, they would have flooded 71,000 acres (290 km²) and destroyed this urban industrial area. More than nine feet of floodwater covered the town of Kaskaskia in 1993 after it overtopped the levee; only the spire of the Catholic church and roof of a nearby shrine rose far above the waters.
The Mississippi River is the second-longest river and chief river of the second-largest drainage system in North America, second only to the Hudson Bay drainage system. From its traditional source of Lake Itasca in northern Minnesota, it flows generally south for 2,340 miles (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 main stem is entirely within the United States; the total drainage basin is 1,151,000 sq mi (2,980,000 km2), of which only about one percent is in Canada. The Mississippi ranks as the thirteenth-largest river by discharge in the world. The river either borders or passes through the states of Minnesota, Wisconsin, Iowa, Illinois, Missouri, Kentucky, Tennessee, Arkansas, Mississippi, and Louisiana.
Monroe County is a county located in the U.S. state of Illinois. According to the 2020 census, it had a population of 34,962. Its county seat and largest city is Waterloo.
Kaskaskia is a village in Randolph County, Illinois. Having been inhabited by indigenous peoples, it was settled by France as part of the Illinois Country. It was named for the Kaskaskia people. Its population peaked at about 7,000 in the 18th century, when it was a regional center. During the American Revolutionary War, the town, which by then had become an administrative center for the British Province of Quebec, was taken by the Virginia militia during the Illinois campaign. It was designated as the county seat of Illinois County, Virginia, after which it became part of the Northwest Territory in 1787. Kaskaskia was later named as the capital of the United States' Illinois Territory, created on February 3, 1809. In 1818, when Illinois became the 21st U.S. state, the town briefly served as the state's first capital until 1819, when the capital was moved to more centrally located Vandalia.
Prairie du Rocher is a village in Randolph County, Illinois, United States. Founded in the French colonial period in the American Midwest, the community is located near bluffs that flank the east side of the Mississippi River along the floodplain often called the "American Bottom". The population was 502 at the 2020 census.
Cahokia was a village in St. Clair County, Illinois, United States. It was located east of the Mississippi River in the Greater St. Louis metropolitan area. As of the 2010 census, 15,241 people lived in the village, a decline from 16,391 in 2000. On May 6, 2021, the village ceased to exist, being incorporated into the new city of Cahokia Heights.
Ste. Genevieve is a city in Ste. Genevieve Township and is the county seat of Ste. Genevieve County, Missouri, United States. The population was 4,999 at the 2020 census. Founded in 1735 by French Canadian colonists and settlers from east of the river, it was the first organized European settlement west of the Mississippi River in present-day Missouri. Today, it is home to Ste. Genevieve National Historical Park, the 422nd unit of the National Park Service.
The Cahokia Mounds State Historic Site is the site of a pre-Columbian Native American city directly across the Mississippi River from modern St. Louis, Missouri. This historic park lies in south-western Illinois between East St. Louis and Collinsville. The park covers 2,200 acres (890 ha), or about 3.5 square miles (9 km2), and contains about 80 manmade mounds, but the ancient city was much larger. At its apex around 1100 CE, the city covered about 6 square miles (16 km2) and included about 120 earthworks in a wide range of sizes, shapes, and functions.
The Illinois Country —sometimes referred to as Upper Louisiana —was a vast region of New France claimed in the 1600s in what is now the Midwestern United States. While these names generally referred to 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 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 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 French name, Pays des Ilinois, means "Land of the Illinois [plural]" and is a reference to the Illinois Confederation, a group of related Algonquian native peoples.
The Kaskaskia River is a tributary of the Mississippi River, approximately 325 miles (523 km) long, in central and southern Illinois in the United States. The second largest river system within Illinois, it drains a rural area of farms, as well as rolling hills along river bottoms of hardwood forests in its lower reaches. The lower reaches of the river have been canalized to allow barge traffic.
Fort de Chartres was a French fortification first built in 1720 on the east bank of the Mississippi River in present-day Illinois. It was used as the administrative center for the province, which was part of New France. Due generally to river floods, the fort was rebuilt twice, the last time in limestone in the 1750s in the era of French colonial control over Louisiana and the Illinois Country.
The Mississippian culture was a Native American civilization that flourished in what is now the Midwestern, Eastern, and Southeastern United States from approximately 800 CE to 1600 CE, varying regionally. It was known for building large, earthen platform mounds, and often other shaped mounds as well. It was composed of a series of urban settlements and satellite villages linked together by loose trading networks. The largest city was Cahokia, believed to be a major religious center located in what is present-day southern Illinois.
Spiro Mounds is an archaeological site located in present-day eastern Oklahoma that remains from an indigenous Indian culture that was part of the major northern Caddoan Mississippian culture. The 80-acre site is located within a floodplain on the southern side of the Arkansas River. The modern town of Spiro developed approximately seven miles to the south.
Philippe François Renault was a French politician, businessman, explorer, metallurgist, and favorite courtier of King Louis XV of France, who left his native Picardy in 1719 for the Illinois Country, Upper Louisiana, in French North America.
Missouri French or Illinois Country French also known as français vincennois, français Cahok, and nicknamed "Paw-Paw French" often by individuals outside the community but not exclusively, is a variety of the French language spoken in the upper Mississippi River Valley in the Midwestern United States, particularly in eastern Missouri.
St. Philippe is a former village in Monroe County, Illinois, United States. The settlement was founded in ca. 1723 by Frenchman, Philip Francois Renault, during the French colonial period. St. Philippe was strategically located near the bluffs that flank the east side of the Mississippi River in the vast Illinois floodplain known as the "American Bottom". The village was located three miles north of Fort de Chartres. Because of many decades of severe seasonal flooding, St. Philippe and the fort were both abandoned before 1765. After the British takeover of this area following their victory in the Seven Years War, many French from the Illinois country moved west to Ste. Genevieve, Saint Louis, and Missouri
The history of St. Louis, Missouri, from prehistory to 1762 was marked by the presence of the Moundbuilder indigenous culture, the explorations of Europeans, and the establishment of French trading posts along the Mississippi River.
Bois Brûlé is one of the eight townships located in Perry County, Missouri, in the United States of America.
The Bois Brule Bottom is an alluvial floodplain in Bois Brule Township in Perry County, Missouri stretching between Bois Brule Creek to the west and the Mississippi River to the east.
Le Grand Champ is is an alluvial floodplain, also called a bottom, extending along the Mississippi River in Ste. Genevieve County, Missouri.
The Kolmer Site is an archaeological site in the far southwest of the U.S. state of Illinois. Located near Kaskaskia and Prairie du Rocher in western Randolph County, it lies at the site of an early historic Indian village from the French period. Because it occupies a critical chronological and cultural position, it has been given national recognition as a historic site.
Coordinates: 38°24′N90°18′W / 38.4°N 90.3°W