Lake Ontario () is one of the five Great Lakes of North America. It is bounded on the north, west, and southwest by the Canadian province of Ontario, and on the south and east by the U.S. state of New York. The Canada–United States border spans the centre of the lake. On the Canadian side, the major cities are Kingston, Mississauga, Toronto, Hamilton, and St. Catharines. On the American side, the major cities are Rochester and Watertown.
As the last in the Great Lakes chain of freshwater lakes, Lake Ontario serves as the outlet to the Atlantic Ocean via the St. Lawrence River, comprising the western end of the St. Lawrence Seaway. Its primary inlet is the Niagara River from Lake Erie. The Long Sault control dam, primarily along with the Moses-Saunders Power Dam regulates the water level of the lake.
Name
The French explorer Samuel de Champlain named the lake "Lac S. Louis" (in English: Lake Saint Louis) in 1632, a name that remained in use until the end of the 17th century.
The name Ontario is likely derived from the Iroquoian word or meaning "lake of shining waters" or "beautiful lake". It has been in use since at least 1641. During French colonial Canada, various other names for the lake were also used, including Frontenac and St. Louis, though Ontario ultimately became the common term.
Amidst an ongoing trade war with Canada, United States president Donald Trump signed an executive order in August 2026 directing US federal agencies to refer to the lake as Lake America. The name was rejected by Canadian officials and numerous US state officials, but was adopted by a number of private companies offering mapping services, including Apple and Google, when serving their U.S. users.
Geography
Lake Ontario is the easternmost of the Great Lakes and the smallest in surface area (), of which about is in Canada and in the United States. Lake Ontario exceeds Lake Erie in volume ( to ). It is the 13th-largest lake in the world. When its islands are included, the lake's shoreline is about long. As the last lake in the Great Lakes' hydrologic chain, Lake Ontario has the lowest mean surface elevation of the lakes at above sea level; lower than its neighbor upstream. Its maximum length is , and its maximum width is . The lake's average depth is 47 fathoms 1 foot (), with a maximum depth of 133 fathoms 4 feet () in the Rochester Basin.
The lake's primary source is the Niagara River, draining Lake Erie, with the Saint Lawrence River serving as the outlet. The drainage basin covers .A Report on Water Resources and Local Watershed Management Programs . The State of the New York Lake Ontario Basin (2000) As with all the Great Lakes, water levels change both within the year (owing to seasonal changes in water input) and among years (owing to longer-term trends in precipitation). These water level fluctuations are an integral part of lake ecology and help create and maintain extensive wetlands.Wilcox, D.A, Thompson, T.A., Booth, R.K., and Nicholas, J.R.. 2007. Lake-level variability and water availability in the Great Lakes. U.S. Geological Survey Circular 1311, 25 p.Keddy, P.A. 2010. Wetland Ecology: Principles and Conservation (2nd edition). Cambridge University Press, Cambridge, UK. Chapter 2. The lake also has an important freshwater fishery, although it has been negatively affected by factors including overfishing, water pollution and invasive species.Christie, W. J. (1974). Changes in the fish species composition of the Great Lakes. Journal of the Fisheries Research Board of Canada, 31, 827–54.
Baymouth bars built by prevailing winds and currents have created a significant number of lagoons and sheltered harbours, mostly near (but not limited to) Prince Edward County, Ontario, and the easternmost shores. Perhaps the best-known example is Toronto Bay, chosen as the site of the Upper Canada capital for its strategic harbour. Other prominent examples include Hamilton Harbour, Irondequoit Bay, Presqu'ile Bay, and Sodus Bay. The bars themselves are the sites of long beaches, such as Sandbanks Provincial Park and Sandy Island Beach State Park. These sand bars are often associated with large wetlands, which support large numbers of plant and animal species, as well as providing important rest areas for migratory birds.Maynard, L., and Wilcox, D.A., 1997, Coastal wetlands of the Great Lakes—State of the Lakes Ecosystem Conference 1996 background paper: Environment Canada and U.S. Environmental Protection Agency, EPA 905–R–97–015b, 99 p.Keddy, P.A. 2010. Wetland Ecology: Principles and Conservation (2nd edition). Cambridge University Press, Cambridge, UK.497 p. Presqu'ile, on the north shore of Lake Ontario, is particularly significant in this regard. One unique feature of the lake is the Z-shaped Bay of Quinte which separates Prince Edward County from the Ontario mainland, save for a isthmus near Trenton; this feature also supports many wetlands and aquatic plants, as well as associated fisheries.
Major rivers draining into Lake Ontario include the Niagara River, Don River, Humber River, Rouge River, Trent River, Cataraqui River, Genesee River, Oswego River, Black River, Little Salmon River, Credit River, Ganaraska River and the Salmon River.
On the Canadian side, the Welland Canal connects Lakes Ontario and Erie and is used for maritime navigation between the two lakes.
Geology The lake basin was carved out of soft, weak Silurian-age rocks by the Wisconsin ice sheet during the last ice age. The action of the ice occurred along the pre-glacial Ontarian River valley, which had approximately the same orientation as today's basin. Material that was pushed southward by the ice sheet left landforms such as drumlins, kames, and moraines, both on the modern land surface and the lake bottom,Origin of drumlins on the floor of Lake Ontario and in upper New York State; Quaternary geology; bridging the gap between East and West — Department of Geology, University of Toronto . Geology.utoronto.ca (November 17, 2011). Retrieved on November 29, 2011. reorganizing the region's entire drainage system. As the ice sheet retreated toward the north, it still dammed the St. Lawrence Valley outlet, so the lake surface was at a higher level. This stage is known as Lake Iroquois. During that time, the lake drained through present-day Syracuse, New York, into the Mohawk River, thence to the Hudson River and the Atlantic. The shoreline created during this stage can be easily recognized by the (now dry) beaches and wave-cut hills from the present shoreline.
When the ice finally receded from the St. Lawrence valley, the outlet was below sea level, and for a short time, the lake became a bay of the Atlantic Ocean, in association with the Champlain Sea. Gradually the land rebounded from the release of the weight of about of ice that had been stacked on it. It is still rebounding about per century in the St. Lawrence area. Since the ice receded from the area last, the most rapid rebound still occurs there. This means the lake bed is gradually tilting southward, inundating the south shore and turning river valleys into bays. Both north and south shores experience shoreline erosion, but the tilting amplifies this effect on the south shore, causing loss to property owners.
Climate
The lake has a natural seiche rhythm of eleven minutes. The seiche effect normally is only about but can be greatly amplified by earth movement, winds, and atmospheric pressure changes. Because of its great depth, the lake as a whole does not completely freeze in winter, but an ice sheet covering between 10% and 90% of the lake area typically develops, depending on the severity of the winter. Ice sheets typically form along the shoreline and in slack water bays, where the lake is not as deep. During the winters of 1877 and 1878, the ice sheet coverage was up to 95–100% of the lake. During the War of 1812, the ice cover was stable enough the American naval commander stationed at Sackets Harbor feared a British attack from Kingston, over the ice. The lake has completely frozen over on five recorded occasions: in 1830,Kingston Chronicle, January 30, 1830, 2, col. 6 ("For several years past we have not been visited with so much snow as has fallen here within the last fortnight. The storm of Wednesday and yesterday could only be equalled by such visitations as are familiar to our Lower Canada friends. The thermometer has ranged from 10° below, to 20° above 0, for the last ten days. The Lake is firmly frozen, and a cheap and safe style of travelling has revived the intercourse with our brethren of the independent portion of the world"); The Republican Compiler [newspaper], February 23, 1830, p. 2, col. 5 ("At Kingston, Upper Canada, the quantity of snow which had fallen had not been equaled for several years.—The Lake (Ontario) was frozen, and crossing had become general"); Perry, Kenneth A, The Fitch Gazetteer: An Annotated Index to the Manuscript History of Washington County, New York, 4 vols. (Bowie, Md.: Heritage Books, 1999), 4:565 ("Kingston, Upper Canada, [experiencing] the deepset snow in several yrs., & Lake Ontario frozen over"); Kingston Chronicle [newspaper], January 9, 1830, 2, col. 1 ("the Bay was frozen across this morning"); see also Vermont Chronicle, (Bellows Falls, Vt.) Friday, February 19, 1830, p. 31, col D, quoting the Quebec Gazette: "The Lake (Ontario) was frozen, and crossing had become general." 1874, 1893, 1912, and 1934.
When the cold winds of winter pass over the warmer water of the lake, they pick up moisture and drop it as lake-effect snow. Since the prevailing winter winds are from the northwest, the southern and southeastern shoreline of the lake is referred to as the snowbelt. In some winters, the area between Oswego and Pulaski may receive twenty or more feet (600 cm) of snowfall. Also impacted by lake-effect snow is the Tug Hill Plateau, an area of elevated land about east of Lake Ontario. The "Hill", as it is often referred to, typically receives more snow than any other region in the eastern United States. As a result, Tug Hill is a popular location for winter enthusiasts, such as snow-mobilers and cross-country skiers. Lake-effect snow often extends inland as far as Syracuse, with that city often recording the most winter snowfall accumulation of any large city in the United States. Other cities in the world receive more snow annually, such as Quebec City, which averages , and Sapporo, Japan, which receives each year and is often regarded as the snowiest city in the world.
Foggy conditions (particularly in fall) can be created by thermal contrasts and can be an impediment for recreational boaters. Lake breezes in spring tend to slow fruit bloom until the frost danger is past, and in the autumn delay the onset of fall frost, particularly on the south shore. Cool onshore winds also slow the early bloom of plants and flowers until later in the spring season, protecting them from possible frost damage. Such microclimatic effects have enabled tender fruit production in a continental climate, with the southwest shore supporting a major fruit-growing area. Apples, cherries, pears, plums, and peaches are grown in many commercial orchards around Rochester. Between Stoney Creek and Niagara-on-the-Lake on the Niagara Peninsula is a major fruit-growing and wine-making area. The wine-growing region extends over the international border into Niagara and Orleans counties in New York. Apple varieties that tolerate a more extreme climate are grown on the lake's north shore, around Cobourg.
Ecology