7,540 sq mi (19,529 sq km), 193 mi (311 km) long and 53 mi (85 km) at its greatest width, between SE Ont., Canada, and NW N.Y.; smallest and lowest of the Great Lakes. It has a surface elevation of 246 ft (75 m) above sea level and a maximum depth of 778 ft (237 m).
Lake Ontario is fed chiefly by the waters of Lake Erie by way of the Niagara River; other tributaries are the Genesee, Oswego, and Black rivers in New York and the Trent River in Ontario. The lake is drained to the northeast by the St. Lawrence River. Oceangoing vessels reach the lake through the St. Lawrence Seaway and use the Welland Canal to bypass Niagara Falls and reach Lake Erie; smaller craft (mostly pleasure boats) can travel the Rideau Canal between Kingston and Ottawa, and the Trent Canal between the Bay of Quinte and Georgian Bay. Navigation on the lake is not usually impeded by ice in winter.
The chief Canadian lakeshore cities are St. Catharines, Hamilton, Toronto, Oshawa, and Kingston; on the south shore are Rochester and Oswego, N.Y. Commercial fishing is important, but pollution has been a problem. A U.S.-Canadian pact (1972) established that water quality would be improved and further pollution ended. Recreational facilities are provided at state and provincial parks. The first European to see (1615) Lake Ontario was étienne Brulé, the French explorer; later that year Samuel de Champlain visited it.