Mo. Rev. Stat. § 46.132: Platte.

Beginning in the middle of the main channel of the Missouri River, at a point where a prolongation south of the old boundary line of the state would intersect the same; thence north with said boundary line to the line run and marked by Matthew M. Hughes, under an act of the general assembly of 1838 and 1839; thence west with said line to the middle of the main channel of the Missouri River; thence down said river, in the middle of the main channel thereof, to the place of beginning.