Lake Ontario is the smallest and most easterly of the Great Lakes of North America. It is bounded on the north by the Canadian province of Ontario and on the south by Ontario’s Niagara Peninsula and the U.S. state of New York. The state of Pennsylvania benefits through being included in its drainage basin.
The recent history of Lake Ontario has been affected by wars: Britain gained control of the Lake during the French and Indian Wars while the American Revolution encouraged trade, shipping and settlement in the area. Ontario’s major urban and industrial centers, the cities of Hamilton and Toronto, are on its shore.
Lake Ontario suffered tremendously from pollution from industrial and agricultural waste. By the 1960s and 1970s the lake was dying. Since that time, environmental concerns have forced a cleanup of these wastes, which has been accomplished through better treatment plants and tighter environmental regulations. Today, Lake Ontario has recovered much of its pristine quality.
The lake served as the border between the Huron and their vassals and the Iroquois Confederacy in pre-European times. The first documented European to reach the lake was Étienne Brûlé in 1615. Artifacts believed to be of Norse origin have been found in the area, indicating possible earlier visits by Europeans.
A series of trading posts was established by both the British and French, such as Fort Oswego in 1722 and Fort Rouillé 1750 (in Toronto). After the French and Indian War, all the forts were under British control. This remained the case even in the years following the American Revolution until the signing of the Jay Treaty in 1794, when forts on the U.S. side of the lake became American. Permanent, non-military European settlement had begun during the American Revolution, preceding settlement on the other Great Lakes. It became a hub of commercial activity following the War of 1812 with canal building on both sides of the border and was heavily traveled by lake steamers, which reached their peak activity in the mid-nineteenth century prior to competition from railway lines.
(Courtesy of http://www.newworldencyclopedia.org/entry/Lake_Ontario)