What is the main source of inflow into Lake Ontario?

The main source of inflow into Lake Ontario is indeed the Niagara River. This river is significant as it feeds Lake Ontario with water from Lake Erie. The flow from Lake Erie through the Niagara River is responsible for a substantial volume of water entering Lake Ontario, making it a crucial hydrological component of the Great Lakes system.

While the Saint Lawrence River does carry water away from Lake Ontario into the Atlantic Ocean, it does not serve as a source of inflow; instead, it acts as an outlet. The Ottawa River and the Great Lakes also do not directly feed into Lake Ontario. Thus, recognizing the role of the Niagara River is essential for understanding the dynamics of Lake Ontario's water levels and hydrology.