Lake Ontario is a large, freshwater lake. It is very deep, 86 metres on average and 245 m at its deepest, but it has the smallest surface area of the Great Lakes (18 960 km2). Nonetheless, it has the largest watershed area relative to its surface area of all the Great Lakes.

Eighty percent of Lake Ontario's water originates in Lake Erie to which it is naturally connected by the Niagara River, made famous by Niagara Falls (a drop of almost 100 metres). The Welland Canal was built to allow ships to go between the two lakes and bypass the falls. The remaining water in Lake Ontario comes from tributaries in its watershed (rivers and streams, 14%) and from precipitation directly on its surface (7%). Lake Ontario's outlet is the St. Lawrence River, to which it loses 93% of its outflowing water. The remaining 7% is lost through evaporation.

Lake Ontario at Confederation Park. Photo by Betty Blashill.

Lake Ontario is also the recipient of materials flowing through the entire Great Lakes chain because of its position at the end of that chain. This means that it is impacted by human activities throughout the entire Great Lakes watershed. Several organisations are now engaged in serious efforts to restore and monitor the Great Lakes ecosystems, including Lake Ontario.

Small Inland Lakes

There are other smaller lakes in the Hamilton area that are mainly the result of river damming, either to control flooding or to regulate stream flow. When a lake is formed in this way it is called a reservoir. Examples of reservoirs in the Hamilton area include the Binbrook Reservoir, Christie Lake, and the reservoir at the Valens Conservation Area. These lakes are managed by the Hamilton Conservation Authority. They are stocked with fish and have chlorinated swimming areas separated from the main water area. Some of them have wetland areas on their margins.

In Halton region, most lakes are also reservoirs, like the Mountsberg Reservoir, but there is one rare, natural lake, , located on top of the Niagara Escarpment called Crawford Lake.

Crawford Lake. Photo courtesy of Conservation Halton.

This lake formed in a deep fissure in the escarpment rock. This fissure is deep enough that the water near the bottom does not get any sunlight and is very cold. This cold bottom layer does not mix with the warmer upper water layer, making this lake 'Meromictic'.