- Altitude: 75 m above sea level
- Surface Area: 19,100 sq. km.
- Mean Depth: 86 m
- Maximum Depth: 245 m
- Volume: 1,640 cu. km
- Shoreline Length: 150 km
- Drainage Basin: 79,600 sq km
Lake Ontario is Canada's seventh largest lake, but is the smallest of the five Great Lakes. Within the Great Lakes, water flows from west to east into the Atlantic Ocean. The major inflow to Lake Ontario is from Lake Erie via the Niagara River. The lake is drained by the St. Lawrence River at the northeast end of the lake. The prevailing westerly summer winds cause a thick epilimnion to develop at the southeastern end of the lake, whereas an absent or thin epilimnion is present in the western end. There are relatively few islands in the lake – those present are found in the eastern Kingston basin. Fish species in Lake Ontario include bullhead, yellow perch, eel, white perch, lake whitefish, sunfish, and carp. Pacific salmon were introduced to the lake for the sport fishery. There was a severe decline in the populations of lake trout and lake whitefish between 1930-1950 because of overfishing, the accidental introduction of sea lamprey, and degradation of spawning grounds.
Eutrophication and toxic contamination have been major pollution problems in Lake Ontario. Lake eutrophication resulted from phosphorus loading associated with rapid urban development in the drainage basin. The 1972 Canada-U.S. Great Lakes Water Quality Agreement has led to a reduction in phosphorus loading by over 50% in the past fifteen years. Contamination with a number of toxic chemicals has caused health warnings for the consumption of fish caught in the lake. Older and fatty fish species have PCB and DDT concentrations above acceptable consumption levels. The toxic effects of these contaminants were particularly evident during the 1960's and 1970's when deformities and population decline of aquatic birds was prevalent. Research has indicated that toxin concentrations have decreased, but the subtle effects of persistent chemicals are poorly understood. Although filtration and sterilization provide safe drinking water from the lake, concern over water quality and human health persists.
Lake Ontario is located on the border between Canada and the United States. On the Canadian side, the most heavily urbanized region in Ontario is located on the west side of the lake. The American side also has an large residential and industrial area, particularly along tributaries and the Niagara River. The lake is utilized as a source of water for municipal and manufacturing purposes as well as power production, transportation, tourism, recreation, and fisheries. Trees characteristic of the watershed include red pine, white pine, maple and basswood in Canada. Few intact tracts of this forest remain, however, and most of the land is used for residential, industrial, or agricultural purposes.