International Lake Ontario - St. Lawrence River Board

Lake Ontario St. Lawrence River Regulation

ISLRBC control image

Water flows from Lake Ontario into the St. Lawrence River and passes through the hydropower project near Cornwall, Ontario and Massena, New York. The International Joint Commission approved this project in 1952. During construction, the Commission amended its order of approval with the concurrence of the United States and Canadian Governments. The 1956 amendments added requirements to reduce the range of Lake Ontario water levels, and to provide dependable flow for hydropower, adequate navigation depths and protection for shoreline and other interests downstream in the Province of Quebec.

One requirement in the Commission’s order was to regulate Lake Ontario within a target range from 74.2 to 75.4 meters (243.3 to 247.3 feet) above sea level. The project must also be operated to provide no less protection for navigation and shoreline interests downstream than would exist without the project. Another provision, known as criterion (k), was included because water supplies would inevitably be more extreme at some time in the future than in the past (1860-1954). When supplies exceed those of the past, shoreline property owners upstream and downstream are to be given all possible relief. When water supplies are less than those of the past, all possible relief is to be provided to navigation and power interests.

Regulatory Facilities

Lake Ontario outflows have been regulated since 1960, primarily through the Moses-Saunders power dam near Cornwall and Massena, about 160 kilometers (100 miles) from the lake. This facility is jointly owned and operated by Ontario Power Generation and the New York Power Authority. Another dam, located near Long Sault, Ontario, acts as a spillway when outflows are larger than the capacity of the power dam. A third structure at Iroquois, Ontario, is principally used to help form a stable ice cover and regulate water levels at the power dam.

The other projects in the St. Lawrence River are not supervised by the Commission. These include three navigation locks in the international section of the St. Lawrence River, two at Massena and one at Iroquois, Ontario, as well as hydropower and navigation facilities downstream in the Province of Quebec. The Commission doesn’t manage the seaway locks because the operation does not affect water levels and flows on a daily basis. The Commission also does not manage the downstream power plants because they are not on the international border and operation does not affect water levels and flows in boundary waters.

Lake Ontario Regulation Plan

The uncontrolled, naturally occurring, water supplies into Lake Ontario are the major factor in determining lake water levels and outflows. The water supplies are inflows from Lake Erie, precipitation minus evaporation over the lake, and runoff from the basin. Plan 1958-D, the current plan, specifies weekly outflows based on the water level of Lake Ontario and the water supplies to the lake. Generally, higher levels and greater water supplies result in higher outflows, and vice versa. The plan has a number of flow limitations to protect various interests in the St. Lawrence River that may be affected by extreme flows or levels. These include adequate flows for hydropower production, minimum depths for navigation, and protection against flooding.

Regulation of Lake Ontario outflows does not ensure full control of Lake Ontario levels or levels downstream. The major natural factors affecting levels (precipitation, evaporation, runoff, inflow from Lake Erie, and wind effects on Lake levels) cannot be controlled. Their prediction is very complex and may not be accurate.

During periods of sustained high or low water supplies, regulation of outflows has helped to make water levels less extreme. During the very low water supply period of the mid-1960s, for example, Lake Ontario levels were maintained higher than they would have been without the project. During the high water supply periods of the early and mid-1970s, mid-1980s, and 1993, water levels were held well below pre-project levels, providing considerable relief to shoreline interests.

At the beginning of winter, outflows are usually reduced to help form an ice cover on the St. Lawrence River. After a stable ice cover forms, flows can be increased to offset any flow reductions. Experience has shown that during spring runoff from the Ottawa River which is a major tributary, flooding in the Montreal area has been reduced by temporary reductions in Lake Ontario outflow.