|(Fig. 1) Original proposed Illinois borders within the Illinois territory. A future addition to Illinois from the future state of Wisconsin.
Nathaniel Pope, Illinois Territory Delegate in the United States Congress, proposed modifying the northern border by moving it 51 miles north for economic reasons and giving Illinois access to Lake Michigan, the Great Lakes, and the St. Lawrence Seaway. Another reason for the northern border move was unstated but was related to slavery. After the Missouri Comprise of 1820, Illinois would become a northern state and a vital part of the Union by 1860. While many in southern Illinois were sympathetic to the Confederate cause during the Civil War (1861-1865), most of the state of Illinois was not.
Many inhabitants living in the northern Illinois Territory (later Wisconsin) objected to the movement of the north boundary, the loss of the Lake Michigan waterfront and the location of a shipping port. The land, water, and population loss delayed Wisconsin's development for 30 years, and Wisconsin finally became a state in 1849. With the help of his brother Senator John Pope of Kentucky, Nathaniel Pope got Congress to move the northern boundary to its present-day location (Fig. 2).
|(Fig. 2) Ancient Mississippi River location east of Quad Cites between the Rock and Green rivers to Illinois River and south to St. Louis. Location of the land additions to Illinois from the future states of Iowa and Missouri.
During the Pleistocene Era (2.6 million years ago until about 11,700 years ago), numerous glacial advances covered most of Missouri and Illinois, with the two most recent designated as the Illinoian and Wisconsinan glaciations. Melt waters from these glaciers contributed to the re-alignment of the Mississippi River. The western boundary of Illinois was the Mississippi River (Fig. 2). However, before the Pleistocene glacial period, the ancient Mississippi River passed much farther to the east, as shown by the blue dashed lines. Today's lower Illinois River follows its course. The Wisconsin glacier eventually blocked the ancient Mississippi River, and its terminal moraine (point of furthest advance southward of a glacier) was about 12,000 to 15,000 years ago. The ancient Mississippi River then re-aligned itself to its current position, later used as the western border when Illinois became a state. If the Mississippi River had not been re-aligned, the 7.5 million acres (Fig. 2) would belong to the conditions of Missouri and Iowa. Before 1803, the French controlled the land west of the current Mississippi River and was part of the Louisiana Purchase that year. After Iowa and Missouri became states, they had a border dispute settled by the US Supreme Court. The border between these two states was primarily the 40°35" latitude line, which, if extended into the current area of Illinois between the Illinois and Mississippi rivers (Fig. 2), would determine the acreage each state would have gained if the ancient Mississippi River had not re-aligned. A total of 3.5 million acres would have gone to Missouri and 4 million acres to Iowa. This area includes some of Illinois's most productive soils for corn and soybean production.
Further to the south, the Mississippi River (just south of current Cape Girardeau, Missouri) was re-routed (Fig. 3) at the end of the Great Ice Age. After the last glacial advance, the melting ice flooded and altered the course of many channels and streams, including the Mississippi and Ohio rivers. Approximately 12 to 15 thousand years ago, scientists believe that the Ohio and Mississippi rivers changed course (Fig. 3) south of Cape Girardeau, Missouri.
|(Fig. 3) The re-alignment of the Mississippi River south of Cape Girardeau, Missouri. The bedrock lined the Mississippi River channel near Thebes, Illinois.
At Thebes, the Mississippi River is now located 30 miles to the east (Fig. 3) of where the ancient Mississippi River flowed. Before the 20th century, the Mississippi River migrated rapidly by eroding the outside and depositing on the inside of a river bend. Numerous oxbow lakes  mark old positions of the channel that have been abandoned. Early Holocene (the term given to the last 11,700 years of the Earth's history) to late Wisconsin liquefaction (conversion of soil into a fluid-like mass during an earthquake or other seismic events) features in western lowlands were induced by a local source, possibly by the Commerce fault (which is north of New Madrid Fault) as a result of earthquake upheaval along the Commerce Geophysical lineament running from central Indiana to Arkansas.
Floodwaters of the ancient Mississippi River did not initially pass through this relatively narrow channel and valley. Instead, they were routed by the bedrock-controlled uplands near Scott City, Missouri, and north of Commerce and Benton, Missouri (Fig. 3) to an opening in the upland ridge 40 miles to the southwest. Then the River turned back to the south and merged with the ancient Ohio River near Morely, Missouri. Once floodwaters of the Mississippi River (from the north) and Ohio River (from the east) could cut a valley trench along a fault and through the bedrock-controlled upland west of Thebes. As a result of the Commerce fault, the distance the Mississippi had to travel was shortened from 50 miles to 6 miles. The two historic rivers also once joined at Malden, Missouri; however, the location of the confluence continued to change over time and is now located south of Cairo, Illinois, at Fort Defiance State Park . The confluence of these two mighty rivers created a very rapidly changing channel. It appears that the bedrock-controlled upland was worn away by both rivers after seismic activity. The creation of the Commerce fault contributed to the opening of the bedrock-controlled channel (Fig. 3) after the last glacial advance, approximately 12,000 years to 15,000 years ago.
The modern-day Cache River Valley of southern Illinois (Fig. 4) has a string of tupelo-cypress (trees) swamps, sloughs, and shallow lakes, remnants of the ancient Ohio River whose confluence with the Mississippi River was once northwest of Cairo, Illinois.
|Cache River Valley on the Ohio River in Illinois.
|(Fig. 4) The location of the ancient Cache River valley and ancient Ohio and Tennessee Rivers.
Hydrologically, the Ohio River is the main eastern tributary of the Mississippi River. Today it runs along the borders of six states 981 miles west from Pittsburgh, Pennsylvania, to the Mississippi River confluence at Cairo, Illinois and drains lands west of the continental divide from the Appalachian Mountains encompassing all or part of 14 states. The Ohio River, a southwestern flowing river, was formed between 2½ and 3 million years ago when glacial ice-dammed portions of north-flowing rivers.
About 625,000 years ago, the ancient Ohio River, fed by Kentucky's Green and Cumberland rivers, flowed through the Cache River Basin and was smaller than the current Ohio River. The Wabash River (Indiana) had yet to form at that time. The Tennessee River was not a tributary of the Ohio River but formed the main channel before the later Ohio River appeared.
During the Woodfordian period (75,000 to 11,000 years ago), the floodwaters from the historic Ohio River watershed drained into eastern Illinois via Bay Creek (Fig. 4) to the northwest and then west through the Cache River Valley through present-day Alexander County, Illinois, where it converged with the Mississippi River near Morely, Missouri, located west of the Horseshoe State Conservation area. The middle Cache River Valley is 1.3 miles wide due to the previous River having been much larger since it carried waters from the ancient Ohio River Valley and the local waters from the upper Cache River Valley to the Mississippi River.
Extensive deposits of gravel and sand, some as deep as 160 feet, rest on the bedrock floor of the middle and eastern portions of the valley and offer evidence of glacial flooding which carved the valley deeply into the bedrock and then, as the water receded, back-filled the valley with sediments. With increasing sediment fill and climate changes, the ancient Ohio River shifted away from the Cache River Valley and into its present course. This event probably took place between 8,000 and 25,000 years ago. As a result, the Cache River became a slow-moving stream with extensive isolated, low swampy areas with a water table that ebbed and flowed with seasonal precipitation.
The upper and middle sections of the Cache River Valley, the Main Ditch, and Bay Creek are located in the ancient Ohio River Valley, where river water crossed through the state of Illinois approximately 10-20 miles north of the present Ohio River position. The Cache River Valley is deeper at a lower elevation (between 320 and 340 feet) than expected in a slow-moving swampy river system. The New Madrid Fault runs under and near Karnak and Ullin, Illinois, and the Cache River Valley elevation does not fit with the rest of the area. Steve Gough, a land-use change-over-time expert, has suggested a large section under the Cache River Valley sank during a significant earthquake in about 900 AD. The cypress trees in the Cache River Valley swamps are up to 1,000 years old, which would be consistent with this time estimate.
|(Fig. 5) The additions and subtractions to Illinois. The orange area is the net border of Illinois without all the Mississippi and Ohio rivers re-routing and the decision to provide Illinois with Lake Frontage on Lake Michigan and connecting waterways.
Compiled by Dr. Neil Gale, Ph.D.
The History of Ancient Lake Chicago and Today's Lake Michigan.
Illinois on the west side of the Mississippi River - Missouri on the east side of the River.
 Just south of Cape Girardeau, Missouri, the Mississippi River cuts a seven-mile gorge through the thick limestone of the Shawneetown Ridge. The gorge, known as the Thebes Gap or the Grand Chain, is as narrow as 3,000 feet in places and was notoriously difficult to navigate.
 An oxbow lake is a U-shaped lake that forms when a wide meander of a river is cut off, creating a free-standing body of water.
 Fort Defiance, known as Camp Defiance during the American Civil War, is a former military fortification located at the confluence of the Ohio and Mississippi rivers near Cairo in Alexander County, Illinois.