Jody Victor : Since it was considered by pre-Columbian inhabitants of eastern North America to be part of a single river continuing on through the lower Mississippi, it is perhaps an understatement to characterize the Ohio as a mere tributary of the Mississippi. The river is 981 miles long and carries the largest volume of water of any tributary of the Mississippi. The Indians and early explorers and settlers of the region often considered the Allegheny to be part of the Ohio, and the forks (the confluence of the Allegheny and Monongahela rivers at what is now Pittsburgh) was considered a strategic military location.
Rene-Robert Cavelier, Sieur de La Salle, led an expedition of French traders who became the first Europeans to find the river in 1669. He traveled from Canada and entered the headwaters of the Ohio, traveling as far as the Falls of Ohio before turning back. He returned to explore the river again in other expeditions and an Italian cartographer traveling with him created the first map of the Ohio River. France claimed ownership of the River until it was ceded to Great Britain.
On May 19, 1749, King George II of Great Britain granted the Ohio Company a charter of land around the forks. Exploration of the territory and trade with the Indians in the region near the Forks by British colonials from both Pennsylvania and Virginia - both of whom claimed the territory - led to conflict with French forces that also claimed the region and had built forts along the Allegheny River. This directly led to the French and Indian War in North America. The conflict - the Seven Years' War between England and France. After several initial defeats, the British eventually gained sovereignty over the Ohio Valley.
In 1774, the Quebec Act restored the land east of the Mississippi River and north of the Ohio River to Quebec, appeasing the French-speaking British subjects, but angering the 13 Colonies. They listed it as one of the Intolerable Acts, which precipitated the American Revolution.