Cognizant that many States and local interests have in the past contributed to the Government, or have contracted to pay to the Government over a specified period of years, money equivalent to the cost of providing for them water storage space at Government-owned dams and reservoirs, constructed by the Corps of Engineers of the United States Army, and that such practices will continue, and, that no law defines the duration of their interest in such storage space, and realizing that such States and local interests assume the obligation of paying substantially their portion of the cost of providing such facilities, their right to use may be continued during the existence of the facility as hereinafter provided.