The Ohio River was the southern boundary of the United States' Northwest Territory. On modern maps, this is identifiable as the southernmost boundaries of the states of Illinois, Indiana and Ohio.
The Northwest Territory became a U.S. territory in 1783 by the Treaty of Paris and was the United States' first territorial possession. It was made up of the region known as the Old Northwest: south and west of the Great Lakes, northwest of the Ohio River and east of the Mississippi River. It included the present states of Ohio, Indiana, Illinois, Michigan and Wisconsin and part of Minnesota. Over the years, from 1800 to 1836, the territory gradually split into states, beginning with Indiana.