Nothing in this subchapter shall be construed as modifying in any manner the existing contract, dated October 23, 1918, between the United States and the Imperial Irrigation District, providing for a connection with Laguna Dam; but the Secretary of the Interior is authorized to enter into contract or contracts with the said district or other districts, persons, or agencies for the construction, in accordance with this subchapter of said canal and appurtenant structures, and also for the operation and maintenance thereof, with the consent of the other users.