Introduction
Lake Ontario is the smallest of the five Great Lakes by surface area at 19,000 square kilometers, yet it is the second-deepest after Lake Superior, reaching a maximum depth of 244 meters. Located at the lowest elevation of the Great Lakes chain, Ontario receives the combined outflow of all four upper lakes through the Niagara River and sends its waters to the Atlantic Ocean via the St. Lawrence River.
The lake is bordered by the Canadian province of Ontario to the north and New York State to the south. Toronto, Canada's largest city with a metropolitan population of over six million, dominates the northwestern shore, while Rochester and other mid-sized cities line the American side. Despite being the smallest Great Lake, Ontario's depth gives it the fourth-largest volume in the system.
Geography & Formation
- Surface Area: 19,000 km²
- Max Depth: 244 m
- Length: 311 km
- Elevation: 75 m above sea level
Lake Ontario sits at just 75 meters above sea level, more than 100 meters lower than Lake Erie. The dramatic elevation drop between the two lakes creates the Niagara Escarpment and Niagara Falls, one of the most famous natural landmarks in the world. The lake basin was carved by glacial ice moving along the relatively soft bedrock of the Ontario Basin during the last glaciation.
The lake's shape is roughly elliptical, oriented on a southwest-northeast axis. The deepest waters are found in the central and eastern portions of the lake, while the western end near Hamilton and Toronto is somewhat shallower. The Thousand Islands region at the lake's eastern outlet marks the beginning of the St. Lawrence River.
Water & Hydrology
- Volume: 1,640 km³
- Average Depth: 86 m
- Retention Time: 6 years
- Outflow: St. Lawrence River
Lake Ontario receives water from the entire Great Lakes watershed upstream, making it the final collection point before water exits to the Atlantic. The lake's water level is regulated by the Moses-Saunders Power Dam on the St. Lawrence River, which balances the needs of shipping, hydropower generation, shoreline property owners, and downstream communities.
The lake rarely freezes completely due to its depth and the moderating influence of its enormous water volume. This keeps the surface temperature relatively mild in winter compared to the surrounding land, creating a lake-effect climate that extends the growing season along its shores. This phenomenon is particularly important for the Niagara wine region and fruit orchards of the Ontario shore.
Ecology & Biodiversity
- Fish Species: Over 80
- Fishery Focus: Chinook salmon, lake trout, brown trout
- Water Quality: Improving but challenged by contaminants
Lake Ontario supports significant recreational and commercial fisheries, with Chinook salmon being the most sought-after sport fish. The lake's deep, cold waters also support populations of lake trout, brown trout, and Atlantic salmon, the latter being the subject of ambitious restoration efforts to re-establish self-sustaining populations.
Historical contamination from industrial sources, particularly around the cities of Hamilton, Toronto, and Rochester, left a legacy of toxic sediments containing PCBs, heavy metals, and other pollutants. Several Areas of Concern designated under the Great Lakes Water Quality Agreement have undergone extensive remediation, though contaminants in lake trout continue to require consumption advisories in some areas.
Cultural Significance
- Major Cities: Toronto (6.2 million metro), Rochester, Kingston
- Historic Route: St. Lawrence Seaway
- Islands: Toronto Islands, Thousand Islands
Lake Ontario has been a vital transportation corridor since long before European contact. Indigenous peoples used the lake and the St. Lawrence River as major trade routes, and the same waterways became central to the colonial economies of New France and British Canada. The opening of the St. Lawrence Seaway in 1959 allowed ocean-going vessels to travel from the Atlantic to the heart of North America.
The Thousand Islands region at the lake's eastern end, where it transitions to the St. Lawrence River, is one of North America's most picturesque destinations. Over 1,800 islands dot the river, ranging from tiny rocky outcrops to substantial landmasses supporting grand homes and castles. Boldt Castle on Heart Island, built by the owner of the Waldorf-Astoria Hotel, is the region's most visited attraction.
Visitor Experience
- Best Season: May to October
- Key Destinations: Toronto Islands, Thousand Islands, Niagara-on-the-Lake
- Activities: Sailing, fishing, cycling, wine touring
The Toronto Islands, a short ferry ride from downtown, offer car-free beaches, gardens, and stunning skyline views. The islands were originally a peninsula that was severed by a storm in 1858 and now provide a green oasis in the shadow of Canada's largest city. Centre Island and Ward's Island are the most popular destinations.
Niagara-on-the-Lake, located where the Niagara River meets Lake Ontario, is one of Canada's most charming towns. Known for its award-winning Shaw Festival theatre, historic main street, and position at the heart of Ontario's premier wine region, the town offers a sophisticated counterpoint to the spectacle of Niagara Falls upstream. The surrounding wine country produces world-class ice wine and Chardonnay.
Key Facts
- Lake Ontario is the smallest Great Lake by surface area at 19,000 km²
- It is the second-deepest Great Lake at 244 meters maximum depth
- The lake sits more than 100 meters lower than Lake Erie, creating Niagara Falls
- Water from all four upper Great Lakes passes through Ontario on its way to the Atlantic
- The lake rarely freezes completely due to its depth and volume
Fun Facts
- Toronto, on Lake Ontario's shore, is the fourth-largest city in North America
- The lake's name comes from the Iroquois word meaning "beautiful lake" or "sparkling water"
- Thousand Islands salad dressing was reportedly invented by a fishing guide in the Thousand Islands region
- The St. Lawrence Seaway allows ships from the Atlantic to reach Duluth, Minnesota — 3,700 km inland
Final Thoughts
Lake Ontario serves as the vital link between the Great Lakes and the wider world, channeling the waters of four lakes through the St. Lawrence River to the Atlantic Ocean. Though the smallest of the Great Lakes by area, its depth, its role in continental transportation, and the great city of Toronto on its shores give Ontario an outsized significance in the Great Lakes system.
