What does Niagara Falls flow between?

Niagara Falls flows between Niagara Falls and Niagara Falls.

The water at Niagara Falls drops -- from the crest of the falls to the bottom of the falls.

The Niagara River flows from Lake Erie to the crest of the falls, over the falls, and thence to Lake Ontario. The Niagara River forms part of the international boundary between Canada and the US.

There are two cities at Niagara Falls, and both are named Niagara Falls. The one in Canada is Niagara Falls, Ontario, and the one in the US is Niagara Falls, New York.