Toronto sits on the shore of Lake Ontario, one of the five Great Lakes, and not on a river. Three rivers cross the city and empty into the lake: the Humber, the Don, and the Rouge. Lake Ontario has an average depth of 86 metres (283 feet), a maximum depth of 244 metres (802 feet), and a retention time of about six years, according to the U.S. Environmental Protection Agency (EPA).
The lake supplies Toronto’s drinking water, sets its shoreline, and has shaped the city’s flooding record. This page gives the physical figures with their sources, describes the three rivers using Toronto and Region Conservation Authority (TRCA) data, and explains what happened when lake levels reached records in 2017 and 2019.
Lake Ontario in figures
The EPA’s “Physical Features of the Great Lakes” table gives these values for Lake Ontario. Some of them differ from other agencies, as noted.
- Average depth: 86 metres (283 feet). Maximum depth: 244 metres (802 feet).
- Volume: 1,640 cubic kilometres (393 cubic miles).
- Water area: 18,960 square kilometres (7,340 square miles) in the EPA table. The U.S. National Oceanic and Atmospheric Administration (NOAA) gives 19,009 square kilometres on its Lake Ontario page, so the area is stated as “about 19,000 square kilometres” in most summaries.
- Land drainage area: 64,030 square kilometres, including the Niagara River.
- Shoreline length including islands: 1,146 kilometres (712 miles).
- Retention time: 6 years.
NOAA reports that 5.6 million people live along the lake, about 2.8 million on each side of the border, and that water leaves the Great Lakes basin through the St. Lawrence River to the Atlantic Ocean.
| Lake | Average depth (m) | Maximum depth (m) | Retention time (years) |
|---|---|---|---|
| Superior | 147 | 406 | 191 |
| Michigan | 85 | 282 | 99 |
| Huron | 59 | 229 | 22 |
| Erie | 19 | 64 | 2.6 |
| Ontario | 86 | 244 | 6 |
Retention time is calculated from lake volume and mean outflow rate, the EPA notes. A shorter time means water is replaced faster. In the table, Lake Erie, the shallowest lake, has the shortest retention time.
The three rivers of Toronto
TRCA, the conservation authority for the Toronto region, describes nine watersheds that drain to Lake Ontario along its 72 kilometres of waterfront, from the Mississauga border in the west to the Ajax and Whitby border in the east. Three of them define Toronto’s geography.
| River | Length | Drainage area | Where it starts and ends (TRCA) |
|---|---|---|---|
| Humber | 126 km | about 90,258 hectares, the largest in TRCA’s area | Niagara Escarpment and Oak Ridges Moraine to Lake Ontario |
| Don | almost 38 km | about 36,000 hectares | Oak Ridges Moraine south to the Keating Channel, where it empties into the lake |
| Rouge | 59 km | about 33,926 hectares | Oak Ridges Moraine to Lake Ontario |
The other six watersheds named by TRCA are Etobicoke Creek, Mimico Creek, Highland Creek, Duffins Creek, Petticoat Creek, and Carruthers Creek. The Humber and the Rouge also carry older routes. TRCA notes that the eastern branch of the Toronto Carrying Place Trail, a link between Lake Ontario and the northern Great Lakes, lies in the Rouge watershed, and the Humber route is the subject of Toronto Carrying Place: history of the Humber portage. Streams that were buried as the city grew are covered in Toronto’s lost rivers and ravines.
Drinking water and the lake
The City of Toronto draws its drinking water from Lake Ontario through intakes between one and five kilometres from shore. The lake’s size, with a volume of about 1,640 cubic kilometres, is the context for the four treatment plants described in Toronto tap water: source, treatment and fluoride.
Record lake levels in 2017 and 2019
The City’s 2024 climate report, prepared by TRCA, records two high-water events. In spring 2017, snowmelt and an exceptionally rainy spring raised Lake Ontario. Impacts on Toronto Island Park began by May 4, and the water level peaked at 75.93 metres (IGLD85) on May 27, which was 0.91 metres above normal averages. Public access to the Island was restricted from May 4 to July 30, 2017, except for about 800 Island residents. Seventy landslides were recorded along the Scarborough Bluffs. On April 14 to 15, 2018, a windstorm produced the highest waves measured in Lake Ontario in at least 45 years, and the combined cost of the 2017 flooding and the 2018 windstorm was about $28 million.
In spring 2019, the level rose again and broke the 2017 record with a peak of 76.03 metres on May 29, 2019. The report puts the cost of shoreline repairs at over $30 million since 2017. The report also notes that maximum ice cover on the lake fell by roughly 3.5 percent per decade between the 1970s and 2010s, and that the lake’s surface water temperature has warmed rapidly since the 1990s. See Toronto Islands: history for the island setting and The Scarborough Bluffs for the eroding shore east of downtown.
Frequently asked questions
Which lake is Toronto on?
Lake Ontario. The City of Toronto draws its drinking water from it, and TRCA describes nine watersheds draining to the lake along the Toronto region’s waterfront.
Is Toronto on a river?
No. Toronto lies on the lake shore. The Humber, Don, and Rouge rivers run through the city and empty into Lake Ontario, according to TRCA.
How deep is Lake Ontario?
The EPA table gives an average depth of 86 metres and a maximum of 244 metres.
How big is Lake Ontario?
About 19,000 square kilometres of water surface. The EPA table lists 18,960 and NOAA lists 19,009, so the exact figure depends on the source and method.
Last verified: October 2026
Sources
- U.S. Environmental Protection Agency, “Physical Features of the Great Lakes” (archived page)
- U.S. Environmental Protection Agency, “Geophysical Lake Ontario” (average depth and retention time) (archived page)
- NOAA, “Lake Ontario,” EcoWatch
- Toronto and Region Conservation Authority, “Humber River”
- Toronto and Region Conservation Authority, “Rouge River Watershed”
- Toronto and Region Conservation Authority, “Don River”
- Toronto and Region Conservation Authority, “Lake Ontario Waterfront”
- TRCA for the City of Toronto, Toronto’s Current and Future Climate, 2024 (PDF)
- City of Toronto, “Tap Water in Toronto”
Related reading
- Toronto Harbour and waterfront: history
- Rouge National Urban Park: history
- Toronto rainfall and storms: climate records
- Where Toronto is: location, province and boundaries
- Toronto facts: location, population, geography and weather
Cite this page
APA: History of Toronto. (2026, October 25). Lake Ontario and the Rivers of Toronto. https://historyoftoronto.ca/blog/lake-ontario-and-toronto-rivers
MLA: “Lake Ontario and the Rivers of Toronto.” History of Toronto, 25 Oct. 2026, https://historyoftoronto.ca/blog/lake-ontario-and-toronto-rivers. Accessed .
Chicago: History of Toronto. “Lake Ontario and the Rivers of Toronto.” Last modified October 25, 2026. https://historyoftoronto.ca/blog/lake-ontario-and-toronto-rivers.
The pages have no named author, so the site is given as the author. Add the date you read the page if your style asks for it.