What is the primary source of water into Lake Ontario?

The primary source of water entering Lake Ontario is Lake Erie, which flows into it through the Niagara River. This connection is crucial because it accounts for the majority of water input into Lake Ontario, making it the most significant contributor to the lake's water levels. The Niagara River is not just a scenic feature; it plays a vital role in the hydrology of the Great Lakes system, allowing for the transfer of water from the upper lakes to Lake Ontario.

It is important to recognize that Lake Huron does contribute water to the Great Lakes system, but this primarily occurs through connections involving the St. Clair River and other tributaries. While rainfall within the Lake Ontario basin does contribute some water, it is significantly less compared to the inflow from Lake Erie. Underground springs can also contribute to a lake's hydrology, but their impact is minimal in comparison to the major rivers and tributaries, particularly the Niagara River in this context.