Reductions in Lake Ontario Outflows Made to Relieve Downstream Flooding

Contact Frank Bevacqua
Fabien Lengellé Washington, D.C.
Ottawa, Ontario (202) 736-9024
(613) 995-0088


Reductions in Lake Ontario Outflows Made
to Relieve Downstream Flooding

Exceptionally warm weather, beginning this past weekend, resulted in rapid snow melt, high inflows and a sudden rise in water levels on Lake Ontario and the St. Lawrence River. Heavy rainfall during the week also compounded the flooding problem in the Montreal area. To provide a measure of relief from the serious flood conditions on Lake St. Louis and elsewhere in the Montreal area, the International St. Lawrence River Board of Control directed a series of large reductions in Lake Ontario outflows.

The rapid snow melt caused record high flows in some of the tributaries to Lake St. Louis and Lake St. Peter near Montreal. Lake St. Louis began to rise abruptly late on March 27 and exceeded flood stage the next day. The spring melt in the lower portion of the Ottawa River basin, which was also brought on by the warm weather, has resulted in high flows to the Montreal area that have come much earlier than usual. Heavy rainfall during the week also exacerbated flooding conditions on Lake St. Louis and brought significant moisture to the Lake Ontario and the Ottawa River basins.

Since late December 1997, the International St. Lawrence River Board of Control has been releasing as much water as possible from Lake Ontario without causing ice jams or flooding downstream. A short ice season this winter has allowed the Board to achieve record outflows during the month of March of 10,200 cubic metres per second (360,000 cubic feet per second), which was the flow until March 27. Beginning on March 28, the series of large flow changes was initiated, which brought the outflow to a record low of 4,700 cms (166,000 cfs) by March 31. In spite of the drastic flow reductions, the levels of Lake St. Louis and the Montreal Harbour rose above flood stage.

The large Lake Ontario outflow reductions also resulted in high water levels on Lake St. Lawrence. To prevent the water from rising above flood stage on Lake St. Lawrence, gates at the Iroquois Dam have been set at an all time minimum open setting.

The level of Lake Ontario has risen nine centimetres (3.5 inches) since March 28 when Lake Ontario outflows were reduced to alleviate flooding in the Montreal area. The lake is presently at 75.26 metres (246.92 feet) above sea level, 50 centimetres (20 inches) above average. The International Joint Commission notes that the flows to Lake Ontario from Lake Erie will remain high for the foreseeable future. Depending on precipitation during the next few weeks, Lake Ontario could rise another 10 to 30 centimetres (four to 12 inches).

The International Joint Commission advises shoreline communities to prepare for the potential continuation of high water levels and other users of the system to prepare for the potential of high flows in the St. Lawrence River.

The Board continues to monitor conditions closely on a continual basis and will adjust the flows as needed to provide all possible relief from high water on Lake Ontario and the St. Lawrence River.

The International Joint Commission was created under the Boundary Waters Treaty of 1909 to help prevent and resolve disputes over the use of waters along the Canada-United States boundary. Its responsibilities include approving certain projects that would change water levels on the other side of the boundary. If it approves a project, the Commission's Orders of Approval may require that flows through the project meet certain conditions to protect interests in both countries. For more information, visit on the Worldwide Web.

The International St. Lawrence River Board of Control was established by the Commission in its 1952 Order of Approval. Its main duty is to ensure that outflows from Lake Ontario meet the requirements of the Commission's Orders. The Board also develops regulation plans and conducts special studies as requested by the Commission. For more information, on the Worldwide Web.

Provisional water level gauge readings, updated every three hours, can be found at on the Worldwide Web.