Lake Ontario is the smallest and easternmost of the Great Lakes, covering only 19,099 km2, but is still nearly 30 times the area of L. Biwa. The Lake forms part of the international boundary between Canada and the United States; the whole northern shore belongs to the Canadian Province of Ontario and its southern shore is in the State of New York. Water flows into the lake from L. Erie through the Niagara River with the famous Niagara Falls, and flows out at the northeast end into the St. Lawrence River. There are no large islands in the lake, apart from the far east corner at the entrance to the St. Lawrence River. The most densely populated and industrialized area of Ontario Province is located on the west side of the lake, but there are only a few cities of significant size on the American side.

The eutrophication of the lake started with the increase of population in the lake's drainage basin at the beginning of this century, and continued to advance until around 1973. It has been shown by scientists that the eutrophication of Lakes Ontario, Erie and Michigan is due to the increase in phosphorus loading, of which the main source is sewage effluents and in particular the domestic use of detergents containing phosphorus. After the Canada. U.S. international treaty was signed in 1972 for preventing eutrophication of the Great Lakes, however, the concentration of phosphorus in the lake water is decreasing owing to the prohibition on detergent use and the required treatment of sewage discharged into the drainage basin.