Location: Lake Ontario is the easternmost and smallest by surface area of the five Great Lakes. It lies between Ontario (Canada) to the north and New York State (USA) to the south. It connects upstream to Lake Erie through the Niagara River and drains eastward into the St. Lawrence River.
Major Cities: Important urban and port centers line the lake’s shores, especially within the Golden Horseshoe region of Canada.
Cities on the Canadian Side: Toronto, Mississauga, Hamilton, Kingston, Oshawa, Burlington, St. Catharines.
Cities on the U.S. Side: Rochester, Oswego, Watertown.
Size: Though the smallest Great Lake by area, Lake Ontario is notably deep.
Surface Area: ~7,300 sq mi (18,960 sq km)
Volume: ~393 cu mi (1,640 cubic km)
Maximum Length: 193 mi (311 km)
Maximum Width: 53 mi (85 km)
Average Depth: 283 ft (86 m)
Maximum Depth: 802 ft (244 m)
Shoreline: ~726 mi (1,161 km), including islands
Formation and History: The lake basin was carved by retreating glaciers roughly 11,000–12,000 years ago during the last Ice Age. An earlier, larger glacial lake known as Lake Iroquois once occupied the region before water levels stabilized into the present form. Indigenous communities lived along its shores for thousands of years before European exploration and settlement. The lake later became strategically important during colonial trade and the War of 1812.
Major Rivers Feeding Lake Ontario: The Niagara River is the dominant source, delivering most of the lake’s inflow from Lake Erie.
United States Side: Niagara River, Genesee River, Oswego River, Salmon River, Black River
Canada Side: Trent River, Humber River, Don River, Cataraqui River
Outflow: St. Lawrence River – Carries water from Lake Ontario to the Atlantic Ocean
Islands: Around 100 islands are found mainly near the eastern end and along the St. Lawrence River.
Wolfe Island (Ontario)
Amherst Island (Ontario)
Toronto Islands (Ontario)
Howe Island (Ontario)
Galloo Island (New York)
Association Island (New York)
Temperature: The lake experiences strong seasonal variation.
Winter Surface: Near 33–36°F (1–2°C)
Summer Surface: 65–75°F (18–24°C)
Deep Water: Remains close to 39°F (4°C) year-round
Wildlife: The shoreline and surrounding wetlands support a wide range of species.
Mammals: Beaver, muskrat, river otter, mink, white-tailed deer
Birds: Bald eagle, osprey, loon, gulls, cormorants, migratory waterfowl
