What is the primary source of water into Lake Ontario?

The primary source of water entering Lake Ontario is indeed from Lake Erie through the Niagara River. This connection plays a significant role in the hydrology of the Great Lakes system. The flow from Lake Erie to Lake Ontario is a natural process that includes the agricultural, industrial, and urban runoff which ultimately drains into the Niagara River, contributing to the water levels of Lake Ontario.

While rainfall and runoff do contribute to Lake Ontario's water levels, the inflow from Lake Erie is the principal and most significant source of water for the lake. Other sources, like Lake Michigan and Lake Huron, involve more complex connections and do not directly feed into Lake Ontario as the Niagara River does. Therefore, the crucial link with Lake Erie makes it the primary source of water for Lake Ontario.