The United States may acquire and hold lands within the limits of this state as sites for forts, magazines, arsenals, dockyards and other needful buildings, or either of them, as contemplated and provided by the Constitution of the United States, which purchase may be made by contract with the owners or as otherwise provided in this title. In like manner, the United States may acquire and hold lands, rights-of-way and material needed in maintaining, operating or prosecuting works for the improvement of rivers and harbors within this state.