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.

These water level fluctuations are an integral part of lake ecology and produce and maintain extensive wetlands.

[11][12] The lake also has an important freshwater fishery, although it has been negatively affected by factors including overfishing, water pollution and invasive species.

[13] Baymouth bars built by prevailing winds and currents have created a significant number of lagoons and sheltered harbors, 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 harbor.

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,[16] 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 means the lake bed is gradually tilting southward, inundating the south shore and turning river valleys into bays.

The seiche effect normally is only about 3⁄4 inch (1.9 cm) but can be greatly amplified by earth movement, winds, and atmospheric pressure changes.

Ice sheets typically form along the shoreline and in slack water bays, where the lake is not as deep.

[18] 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.

Also impacted by lake-effect snow is the Tug Hill Plateau, an area of elevated land about 20 miles (32 km) east of Lake Ontario.

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.

Apple varieties that tolerate a more extreme climate are grown on the lake's north shore, around Cobourg.

Eutrophication may accelerate this process by providing nitrogen and phosphorus for the more rapid growth of competitively dominant plants.

In the 17th century, reports of an alleged creature named Gaasyendietha, similar to the so-called Loch Ness Monster, being sighted in the lake.

The creature is described as large with a long neck, green in colour, and generally causes a break in the surface waves.

In the late 19th and early 20th centuries, a type of scow known as a stone hooker was in operation on the northwest shore, particularly around Port Credit and Bronte.

Stonehooking was the practice of raking flat fragments of Dundas shale from the shallow lake floor of the area for use in construction, particularly in the growing city of Toronto.

[45] A large conurbation called the Golden Horseshoe occupies the lake's westernmost shores, anchored by the cities of Toronto and Hamilton.

Ports on the Canadian side include St. Catharines, Oshawa, Cobourg and Kingston, near the St. Lawrence River outlet.

The American shore is largely rural, with the exception of Rochester and the much smaller ports at Oswego and Sackets Harbor.

The city of Syracuse is 40 miles (64 km) inland, connected to the lake by the New York State Canal System.

The government of Ontario, which holds the lakebed rights of the Canadian portion of the lake under the Beds of Navigable Waters Act,[49] does not permit wind power to be generated offshore.

[50] In Trillium Power Wind Corporation v. Ontario (Natural Resources),[49] the Superior Court of Justice held Trillium Power—since 2004 an "Applicant of Record" who had invested $35,000 in fees and, when in 2011 the Crown made a policy decision against offshore windfarms, claimed an injury of $2.25 billion—disclosed no reasonable cause of action.

The Great Lakes once supported an industrial-scale fishery, with record hauls in 1899; overfishing later blighted the industry.