Section 6. No lands reserved and excepted of the lands granted to this state by the United States, actually or prospectively valuable for the development of water powers or power for hydro-electric use or transmission, which shall be ascertained and designated by the secretary of the interior within five years after the proclamation of the president declaring the admission of the state, shall be subject to any disposition whatsoever by the state or by any officer of the state, and any conveyance or transfer of such lands made within said five years shall be null and void.