Upon the ratification of this act and a similar act of the state of Missouri by the congress of the United States, the center of the channel of the Missouri river, as its flow extends from its intersection with the fortieth (40th) parallel, north latitude, southward to the middle of the mouth of the Kansas or Kaw river, shall be that portion of the true and permanent boundary line between the states of Missouri and Kansas, subject only to changes which may occur by the natural processes of accretion and reliction, but not by avulsion.