Beginning at a point in the middle of the main channel of the Mississippi River, opposite the mouth of Cuivre River; thence up said river, in the middle of the main channel thereof, to a point opposite the mouth of Eagle Fork; thence up Eagle Fork, in the middle of the main channel thereof, to the line of the fifth principal meridian; thence south along said meridian to the middle of the main channel of the Missouri River; thence down the Missouri River, in the middle of the main channel thereof, to the middle of the main channel of the Mississippi River; thence up the Mississippi River, in the middle of the main channel thereof, to the place of beginning.