The mouth of the Kansas River in the West Bottoms area of Kansas City (at a longitude of 94 degrees 36 minutes West) was the basis for Missouri's western boundary from Iowa to Arkansas when it became a state in 1821 (Kansas entered the Union in 1861.) South of the Missouri River, that longitude still remains the boundary between Kansas and Missouri. North of the Missouri River, the state of Missouri extended its boundary further to the west in 1836 with the Platte Purchase. The river has moved slightly since this designation, but the state boundary has remained the same. This line is known as the Osage Boundary.