I'm pretty sure they mean Lake Ontario. It's been a while since I've been there. But I believe the water falls from Lake Ontario into Lake Eerie.
The Niagara River flows north from Lake Erie to Lake Ontario, forming part of the border between Ontario, Canada, to the west, and New York, United States, to the east. [...]
The river [...] is approximately 58 kilometres (36 mi) long and includes the Niagara Falls.