The authority shall designate certain sections of main transmission lines and facilities in the state for delivery of Colorado river power, including lines to be constructed or acquired by the authority and any transmission lines and facilities which may become available through the United States or an agency thereof, or other public or private sources, to the extent that the authority may be able to arrange for the use of the lines and facilities, as to which sections a uniform rate for power at transmission voltage shall also be made applicable. The sections shall initially include all portions of the lines between the points of generation on the Colorado river and distribution centers where substantial power loads exist as established by the authority.