Beginning at a point in the middle of the main channel of the Mississippi River, where the prolongation east of the subdivisional line crossing and dividing equally township sixty-three would intersect the same; thence west with the subdivisional lines to the southwest corner of section eighteen, township sixty-three, north, range nine, west; thence north with the range line between ranges nine and ten to the northern boundary line of this state; thence east on said boundary line to the middle of the main channel of the Des Moines River; thence down said river, in the middle of the main channel thereof, to the middle of the main channel of the Mississippi River; thence down the middle of the main channel of the Mississippi River to the place of beginning.