Provided, That the United States, its permittees and licensees shall not be responsible or held liable or incur any liability for the damage, destruction, or loss of any mining claim, mill site, facility installed or erected, income, or other property or investments resulting from the actual use of such lands or portions thereof for power development at any time where such power development is made by or under the authority of the United States, except where such damage, destruction, or loss results from the negligence of the United States, its permittees and licensees.