Which major waterfall powered the hydroelectric project using Tesla's AC system?

The major waterfall that powered the hydroelectric project using Tesla’s AC system was Niagara Falls.

The Edward Dean Adams Power Plant was built at Niagara Falls, New York, to harness the enormous flow of the Niagara River. Its first large-scale generating station began operating in 1895 and used alternating-current technology based on Tesla’s polyphase system.

Water was diverted through a canal and a long underground tunnel to reach the turbines. The generators converted the turbines’ mechanical energy into electricity, which could then be transmitted at higher voltages over useful distances. This helped demonstrate why AC was well suited to large regional power networks.

The project is often simplified as electricity being generated directly by the falling water. More precisely, the plant used diverted river water and the hydraulic head associated with Niagara Falls to spin turbines. The original plant eventually supplied industries in Niagara Falls and helped send power toward Buffalo, making the falls a major symbol of modern hydroelectric power.