Beginning at a point in the middle of the main channel of the Missouri River, where the prolongation west of the line run and marked by M. M. Hughes, under the act of the general assembly of 1838 and 1839; thence east with said line to the old boundary line of the state; thence north with said line to the point where it is intersected by the subdivisional line between sections twenty-one and twenty-eight of township fifty-eight, range thirty-three, west; thence west with the subdivisional lines 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.