A new £1m reservoir opened at Killylane in June 1964 to supply water to Larne, Ballyclare, Antrim and Ballymena.
The project at Shane’s Hill took 14 years to complete.
Unveiling a commemorative plaque to mark the official opening, Prime Minister Terence O’Neill said: “The project is a development of real significance enabling a water supply to 60,000 people over an area of 280 sq miles.”
He reported that the reservoir contained almost 300 million gallons of water.
Water was supplied by four tributary streams of the Glenwherry River.
The Prime Minister went on to say that improvements to Northern Ireland’s water and sewerage network since the end of World War II was “dramatic” and involved “extremely large expenditure.