Section 2: Effects of Regulation on Levels and Flows

2.1 What was the purpose of the hydropower and seaway project?  

The main objectives of the St. Lawrence River hydropower and seaway projects are dependable water flows for hydropower generation, sufficient water levels and flows for seaway navigation, and flood reduction both upstream and downstream of the projects. These are reflected in a series of criteria and requirements in the IJC's Order of Approval allowing for the construction and operation of the project.

2.2 What caused changes in the levels of Lake Ontario before regulation, and what is causing changes after regulation?

The same natural factors that changed water levels in the lake before regulation continue to influence the levels after regulation. These natural factors include inflow from Lake Erie and weather patterns (precipitation, wind, and temperatures) collectively driving the hydrological system. Short-term adjustments to the outflows of Lake Ontario have little impact on the water level of that lake in comparison to these natural factors.

2.3 What was the “natural” annual cycle of levels in Lake Ontario before regulation in comparison with after regulation?   

Over the years, the pattern of an early summer peak and an early winter low has been continued, on average, before and after regulation. Annual variations in the hydrologic cycle can shift the timing of these highs and lows. The average fluctuations from winter lows to summer highs is similar; prior to regulation the range was about 0.49 m (1.6 ft), since regulation began in 1960 the ranges has averaged about 0.50 m (1.6 ft). Also, since regulation began, the drop of the high summer water level to the low winter level has been slightly more rapid on average.

2.4 Why were levels on Lake Ontario lower prior to regulation than they have been since regulation?

Water levels of Lake Ontario were, on average, lower before regulation, because water supplies were, on average, less at that time. Since 1960, Lake Ontario would have set new record high water levels several times without regulation. While regulation has actually reduced the impact of higher supplies, it has not completely eliminated it. As an example, during 2017, the lake peaked at 75.88 m (248.95 feet) with regulation. The level would have reached approximately 76.06 m (249.54 feet), approximately 18 cm (7.1 inches) higher than what would have been experienced if the regulation structures and procedures were not in place.

By contrast, during the dry years of the mid 1960s, Lake Ontario would have been approximately 60 cm (2 feet) lower at times than it was with regulation. While the above examples show that the result of regulation during extreme water supply conditions can be significant, in general, during more normal water supply periods, the difference in water levels on Lake Ontario due to regulation is in the order of centimeters/inches and is typically not significant.

2.5 What has caused changes in levels of the St. Lawrence River since regulation?  

The St. Lawrence River starts at the mouth of Lake Ontario and flows to the Gulf of St. Lawrence in the Atlantic Ocean. The effects of the regulation of Lake Ontario reach as far downstream as Three Rivers, Quebec, where tidal effects become more dominant. The International border between Canada and the U.S.A lies between the banks of the river until the Moses-Saunders Power Dam at Cornwall, Ontario and Massena, New York. About 10 km (6 miles) downstream of that point, the river is wholly within Canada, although the drainage basin also extends into the US.

Natural factors such as precipitation, runoff, and surge effects from strong winds continue to influence water levels in the St. Lawrence River and its tributaries after regulation, as they did before. The greatest tributary is the Ottawa River, which has a large influence on the levels and flows of the St. Lawrence River downstream of its junction with the river at Lake St. Louis. With regulation, the outflows from Lake Ontario may be lowered during periods of high Ottawa River flows.

As with all large dams, the levels upstream of the dam in Lake St. Lawrence are primarily determined by the flow rate through the dam, with high flow rates resulting in lower levels and low flow rates resulting in higher levels. In addition to being affected by river flows, levels are influenced when strong winds blow the water in a surge effect anywhere along the river.

2.6 What were the “natural” levels in the St. Lawrence River downstream of Cornwall/Massena before regulation?   

Prior to regulation, the St. Lawrence River downstream of Cornwall, ON and Massena, N.Y. experienced extreme level and flow fluctuations correlating with the fluctuating water level on Lake Ontario. These fluctuations were moderated to some extent by the rapids that used to be located in the river. For areas downstream of Lake St. Francis, St. Lawrence River flows were also influenced by fluctuations in the Ottawa River outflow. The most extreme fluctuations, however, were due to the frequent occurrence of ice jams in the river. The regulatory ability and ice-management practices of the Moses-Saunders project and Hydro-Quebec operations of the Beauharnois complex have essentially eliminated the risk of flooding from ice jams. As the project may reduce outflows from Lake Ontario to counter large flows in the Ottawa River, flooding downstream of its confluence with the St. Lawrence River near Montreal has also been reduced.

2.7 What actions does the ILO-SLRB take to manage ice conditions in the St. Lawrence River during the winter?  

Regulation of Lake Ontario outflows has greatly reduced the incidence of ice jams in the St. Lawrence River, both upstream and downstream of the Cornwall/Massena area. Prior to regulation, the frequent occurrence of ice jams in the river was a major cause of extreme fluctuations in water level and flooding of adjacent shoreline properties.

During the winter, the Board, in conjunction with their Operations Advisory Group, monitors ice formation in the St. Lawrence River closely. Outflows from Lake Ontario can be increased or decreased, as conditions require, for ice management purposes. For example, Lake Ontario outflows may be decreased, in accordance with the I-limit rules of Plan 2014, to reduce the flow velocity and accommodate the formation of a stable ice cover. A stable ice cover helps prevent unconsolidated ice from accumulating at narrow points and obstructions in the river and causing ice jams and associated flooding. A stable ice cover also prevents unconsolidated ice from flowing into and clogging hydropower intakes. Alternatively, in some cases, outflows can be increased to help break up and flush unconsolidated ice that has become caught or that may pose problems at certain locations. After events such as these, outflows from Lake Ontario are adjusted when opportunity arises, to return lake water levels to what they would have been if the outflows had remained as those specified by the regulation plan.

On the St. Lawrence River, the ice formation process usually begins in the lower portion of the river, just upstream of the Montreal area in the Beauharnois Canal, followed by formation upstream of Moses-Saunders Power Dam through the International Section of the river. When a stable ice cover has formed far enough upstream in the International Section, the Iroquois Dam gates may also be lowered slightly to further assist ice formation from this point further upstream towards Lake Ontario. Ice booms are also used in the river to assist in ice formation.

2.8 Why is Lake St. Lawrence low when flows are high? What are the benefits and/or liabilities of such a situation?  

Lake St. Lawrence, immediately upstream of the Cornwall/Massena dam, empties when flow through the dam is increased, as more water from Lake Ontario is released. The resultant lower head, the difference in water elevation through the power dam, may reduce the amount of electricity generated, if not off-set by the increase in flow. At such times, the low water levels, coupled with the higher velocity of the increased flow, make recreational boating more dangerous. This phenomenon is present for forebays located upstream of such water-control dams and should be planned for when siting docks, launches, and marinas. Lake St. Lawrence levels are also affected by the level of Lake Ontario, so Lake St. Lawrence levels can get extremely low during periods of low to normal levels on Lake Ontario when water supplies to Lake Ontario are extremely high, which was the cause of the record low levels experienced in 2018.

2.9 How does regulation of Lake Ontario mitigate spring flooding conditions in the Montreal area?

Montreal has been prone to flooding historically, because it is located at the confluence of the Ottawa and St. Lawrence Rivers. Regulation of Lake Ontario outflows has reduced spring flooding in the Montreal area. The spring runoff from the Ottawa River basin may be very significant and it can increase tenfold in hours. Timely reductions of Lake Ontario outflows during this high-runoff period have repeatedly helped avoid serious flooding in the Montreal and Lake St. Louis areas, such as during the record flooding in the Ottawa River basin in 2017 and 2019. The temporary storage of water on Lake Ontario results in higher plan-prescribed outflows from Lake Ontario following this period, which helps compensate for the reduced outflow during times of high runoff. In addition, as noted above, ice control in the river, which has been facilitated by the project, has significantly reduced the occurrence of flooding due to ice jams.