As a consequence of the French defeat in the Seven Years' War, the Illinois Country east of the Mississippi River was ceded to the British, and the land west of the river to the Spanish. Following the British occupation of the left bank (when heading downstream) of the Mississippi in 1764, some Canadien settlers remained in the area, while others crossed the river, forming new settlements such as St. Louis.
Because of the deforestation that resulted from the cutting of much wood for fuel during the 19th-century age of steamboats, the Mississippi River became more shallow and broad, with more severe flooding and lateral changes in its channel in the stretch from St. Louis to the confluence with the Ohio River. As a consequence, many architectural and archaeological resources were lost to flooding and destruction of early French colonial villages originally located near the river, including Kaskaskia, St. Philippe, Cahokia, and Ste. Genevieve.
The boundaries of the Illinois Country were defined in a variety of ways, but the region now known as the American Bottom was nearly at the center of all descriptions. One of the earliest known geographic features designated as Ilinois was what later became known as Lake Michigan, on a map prepared in 1671 by French Jesuits. Early French missionaries and traders referred to the area southwest and southeast of the lake, including much of the upper Mississippi Valley, by this name. Illinois was also the name given to an area inhabited by the Illiniwek. A map of 1685 labels a large area southwest of the lake les Ilinois; in 1688, the Italian cartographer Vincenzo Coronelli labeled the region (in Italian) as Illinois country. In 1721, the seventh civil and military district of Louisiana was named Illinois. It included more than half of the present state, as well as the land between the Arkansas River and the line of 43 degrees north latitude, and the country between the Rocky Mountains and the Mississippi River. A royal ordinance of 1722—following the transfer of the Illinois Country's governance from Canada to Louisiana—may have featured the broadest definition of the region: all land claimed by France south of the Great Lakes and north of the mouth of the Ohio River, which would include the lower Missouri Valley as well as both banks of the Mississippi.
This boundary between Canada and the Illinois Country remained in effect until the Treaty of Paris in 1763, after which France surrendered its remaining territory east of the Mississippi to Great Britain. (Although British forces had occupied the "Canadian" posts in the Illinois and Wabash countries in 1761, they did not occupy Vincennes or the Mississippi River settlements at Cahokia and Kaskaskia until 1764, after the ratification of the peace treaty.) As part of a general report on conditions in the newly conquered lands, Gen. Thomas Gage, then commandant at Montreal, explained in 1762 that, although the boundary between Louisiana and Canada wasn't exact, it was understood the upper Mississippi above the mouth of the Illinois was in Canadian trading territory.
Distinctions became somewhat clearer after the Treaty of Paris in 1763, when Britain acquired Canada and the land claimed by France east of the Mississippi and Spain acquired Louisiana west of the Mississippi. Many French settlers moved west across the river to escape British control. On the west bank, the Spanish also continued to refer to the western region governed from St. Louis as the District of Illinois and referred to St. Louis as the city of Illinois.