Beginning at a point in the middle of the main channel of the Missouri River, where the prolongation south of the line between ranges twenty-nine and thirty would intersect the same; thence north with said range line to the northeast corner of section thirty-six, township fifty-four, range thirty; thence west with the subdivisional lines to the old state line; thence south with said line to the middle of the main channel of the Missouri River; thence down the Missouri, in the main channel thereof, to the place of beginning.