The Northwest Ordinance of 1787 designated the Ohio River as the southern boundary of the Northwest Territory.
Congress enacted the ordinance on July 13, 1787, to organize the lands north and west of the Ohio River. The territory extended north toward the Great Lakes and west toward the upper Mississippi River. It later became the basis for states including Ohio, Indiana, Illinois, Michigan, and Wisconsin.
The ordinance created a process for territorial government and eventual statehood. It also prohibited slavery in the Northwest Territory, making the Ohio River an important geographic divide between free and slave regions during the early national period. The boundary was not identical to the later Mason-Dixon line, but it became a major political and geographic reference.
The ordinance also promised religious tolerance, trial by jury, and encouragement of education. Its provisions established a model for incorporating new states rather than allowing older states to control western lands permanently. The First Congress reaffirmed the ordinance in 1789 with limited changes.