Beginning in the middle of the main channel of the Missouri River, at a point where the prolongation south of the range line between ranges twenty-five and twenty-six would intersect the same; thence north with said range line to the northwest corner of township fifty-five, range twenty-five, west; thence east with the township line between townships fifty-five and fifty-six to the middle of the main channel of Grand River; thence down said river, in the middle of the main channel thereof, to the middle of the main channel of the Missouri River; thence up said river, in the middle of the main channel thereof, to the place of beginning.