International Lake Ontario - St. Lawrence River Board

Section 2: Effects of Regulation on Levels and Flows

                                                                                                                                                                                                    

2.1 What was the purpose of the hydropower and seaway project?
2.2 What caused changes in the levels of Lake Ontario before regulation, and what is causing changes after regulation?
2.3 What was the “natural” annual cycle of levels in Lake Ontario before regulation in comparison with after regulation?
2.4 Why were levels on Lake Ontario lower prior to regulation than they have been since regulation?
2.5 What has caused changes in levels of the St. Lawrence River since regulation?
2.6 What were the “natural” levels in the St. Lawrence River downstream of Cornwall/Massena before regulation?
2.7 How is the long-term, lake-wide average level on Lake Ontario computed?
2.8 How does the Plan-specified level differ from the actual level on Lake Ontario?
2.9 Why can’t the Lake Ontario level be kept at the average level all year round?
2.10 What actions does the ISLRBC take to manage ice conditions in the St. Lawrence River during the winter?
2.11 Why is Lake St. Lawrence low when flows are high? What are the benefits and/or liabilities of such a situation?
2.12 Why are water levels so variable on the St. Lawrence to the west of Moses-Saunders Dam and so low the winter of 2015? Don’t these low levels harm fish?
2.13 How does regulation of Lake Ontario mitigate spring flooding conditions in the Montréal area?

 

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2.1 What was the purpose of the hydropower and seaway project? [top of page]

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 Orders 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? [top of page]

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? [top of page]

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? [top of page]

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 the wet years of the mid 1980s shown in the above graph, the lake surpassed 75.3 m (247.05 feet) with regulation. The level would have reached approximately 76.0 m (249.34 feet), approximately 70 cm (over 2 feet) 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 as shown in the following graph. 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?  [top of page]

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 Trois Rivieres, 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 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 U.S.A.

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 more controlled 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?  [top of page]

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 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 Montréal has also been reduced.

2.7 How is the long-term, lake-wide average level on Lake Ontario computed?  [top of page]

The long-term , lake-wide, average water level of Lake Ontario is calculated from records of water levels recorded at a coordinated network of gauges strategically located around the Lake Ontario shoreline (URL link to map of the gauges) so as to balance the short-term impacts of winds and barometric pressure. The long-term, coordinated period of record for Lake Ontario begins in 1918, and as such, it includes levels from both before and after the construction of the Moses–Saunders dam. Since lake levels vary naturally on a seasonal basis, the average level of the lake is computed for various time periods throughout the year (e.g., monthly, quarter-monthly, beginning-of-month).

2.8 How does the Plan-specified level differ from the actual level on Lake Ontario?  [top of page]

The Plan-specified level of Lake Ontario is the level at which Lake Ontario would be had the Board previously released outflows strictly in accordance with the regulation plan (Plan 1958-D). The Plan-specified level is calculated from the difference between the actual recorded level of Lake Ontario and the effect of any flow deviations (i.e., differences between actual and Plan-prescribed flows) that may have accumulated. When under-discharge deviations have been incurred (i.e., the Board released flows less than prescribed by Plan 1958-D), water is stored on the lake, and therefore the actual level will be higher than the Plan-prescribed level; whereas when over-discharge deviations have been incurred (i.e., the Board released flows greater than prescribed by Plan 1958-D), water is removed from the lake, and therefore the actual level will be lower than the Plan-prescribed level. The Plan-specified level is equal to the actual level of the lake whenever no deviations have been accumulated.

2.9 Why can’t the Lake Ontario level be kept at the average level all year round?  [top of page]

The level of Lake Ontario is influenced by a number of factors. The drivers of water level fluctuations are primarily the inflow from Lake Erie (which is dependent on water supplies this lake receives as well as those received by lakes further upstream) and, to a lesser extent, the net water supplies of Lake Ontario itself (i.e., the net effects of precipitation, runoff, evaporation, and groundwater). A lesser factor is the outflow released ( explained below). To always maintain Lake Ontario levels at their average values, the regulated outflow from the lake would have to be continually adjusted to compensate for above or below-average water supplies – a near impossible task. It is not possible to measure water supplies with exact certainty, so adjusting outflows to account for water supply variations would be subject to errors. Secondly, and importantly, variations in outflow have a disproportionate impact on water levels in the lower St. Lawrence River compared to Lake Ontario itself, such that substantial increases and decreases in outflow can cause undue harm downstream, while causing little impact on the water levels of Lake Ontario. Furthermore, the outflow from the lake is limited by the physical capacity of the St. Lawrence River and the capacity of the Moses-Saunders hydroelectric dam. The range in capacity is narrower than the range in water supplies the lake has historically received over short time periods. For example, weekly outflows could not be set as high as the highest weekly water supplies that have been, and could again be, experienced. In such cases, Lake Ontario levels will increase. Nor could weekly outflows be set as low as the lowest water supplies that could be experienced, in which case Lake Ontario levels would decrease. Finally, natural water level fluctuations (both highs and lows) are important to ensure a healthy ecosystem. Outflows must be set in consideration of all interests, upstream and downstream, to prevent undue harm to any one interest for the benefit of any other. The regulation plan for Lake Ontario tries to keep water levels between an established upper and lower limit by adjusting outflows depending on the level of Lake Ontario and water supplies received. However, for the reasons outlined, the impact of regulated outflows on water levels is limited, and water levels cannot be kept at or near average values at all times. As a result, during times of relatively dry water supply conditions, the level of Lake Ontario will be lower than the long-term average, whereas the level will be higher during periods of relatively wet water supplies. While the Lake Ontario water level may be close to average at any given time (typically during long periods of relatively normal water supply conditions), rarely is the actual level of the lake the same as the average level for a certain time of year.

2.10 What actions does the ISLRBC take to manage ice conditions in the St. Lawrence River during the winter?  [top of page]

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 below the Plan-prescribed flow 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 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 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.

2.11 Why is Lake St. Lawrence low when flows are high? What are the benefits and/or liabilities of such a situation?  [top of page]

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 headwater lakes located behind water control dams and should be planned for when siting docks, launches, and marinas.

2.12 Why are water levels so variable on the St. Lawrence to the west of Moses-Saunders Dam and so low the winter of 2015? Don’t these low levels harm fish?  [top of page]

The causes and impacts of fluctuating water levels and recent low water conditions in the St. Lawrence River are complex. The section of the river located just west and upstream of the Moses-Saunders Dam and near the villages of Long Sault and Ingleside, is known as Lake St. Lawrence. This is an artificial reservoir, created in the late 1950s by the damming of the river. As such, the reach acts as a headpond for the Moses-Saunders hydroelectric power plant. First let’s explain the causes of the fluctuations and then discuss the impacts on fish.

Lake St. Lawrence levels typically follow a seasonal cycle, rising to seasonal highs in the spring each year, before declining to seasonal lows in the winter. Nonetheless, in 2015 Lake St. Lawrence levels were indeed near historical lows, but they were at similar low levels at the start of December in 1996, and about the same as in early October of 2015. The last time levels were lower than this, at the start of December, was in 1974.

Learning Modules elsewhere on our website, http://www.ijc.org/en_/islrbc/home, under the multimedia tab, explain causes of some of the fluctuations on Lake St Lawrence water levels. Module 2 explains the effect that Lake Ontario outflows, regulated at the dam, have on water levels of Lake St. Lawrence, and Module 6 describes the effects of wind on the water levels. Additional details are provided below, describing the plots of the water levels of Lake Ontario and Lake St. Lawrence, as well as one of Lake Ontario outflows to help better illustrate conditions in 2015, and how they are related.

 

The Board is responsible for setting the outflow from Lake Ontario, ensuring that the outflows follow the Orders of the International Joint Commission, as explained in the FAQs under the News and information tab on our website. In general, when outflows are increased, levels in Lake St. Lawrence are drawn down. In April through June 2015, outflows and Lake Ontario levels were low and consequently levels on Lake St. Lawrence were above average. Outflows were increased following generally wet conditions later this summer on both the Lake Ontario basin, and also the Lake Erie basin (which provides approximately 80% of Lake Ontario’s net total supply of water). Outflows have since remained relatively high, as the net total water supplies into Lake Ontario have also continued to be relatively high.

In addition to the effects of outflows and water supplies, levels of Lake St. Lawrence also experience short-term fluctuations due to the effects of winds. Winds from the southwest push water towards the dam, whereas winds from the northeast direction push water away from the dam. As a result, it is not unusual to see water levels rise and fall sharply during strong, sustained wind events. Such wind events are frequently experienced this time of year. In fact, the large fluctuations observed recently on Lake St. Lawrence, have been largely due to winds, with the extreme low levels being caused by a combination of strong, persistent north-east winds and the continuing relatively high outflows, for this time of year. Lake St. Lawrence water levels typically get quite a bit lower than the current levels, later in the winter.

On average, water levels in February tend to be less than those experienced recently. In fact, during both the previous two years, cold winter conditions resulted in water levels in February being extremely low (largely due to ice). While these low levels can cause difficulties to recreational anglers, fortunately the ecosystem impacts are not as critical at this time of year, when levels are at their seasonal low. Walleye, perch and pike - the main species in Lake St. Lawrence - all spawn in spring, when levels are typically much higher than they are currently, and these fish tend to move to deeper water later in the year. The Board must ensure that the effects on all interests throughout the Lake Ontario – St. Lawrence River system are considered when setting outflows. For example, the November 2015 low Lake St. Lawrence levels impacted commercial navigation in this stretch of the St. Lawrence Seaway, and earlier in that fall, the low levels had impacts on recreational boaters. While a reduction in outflows would raise levels on Lake St. Lawrence, it would result in reduced levels downstream on the St. Lawrence River along with creating possible impacts on commercial navigation near Montreal, and higher levels on Lake Ontario, which may increase risk to shoreline riparians.

2.13 How does regulation of Lake Ontario mitigate spring flooding conditions in the Montréal area?  [top of page]

Montréal 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 Montréal area. The spring runoff from the Ottawa River basin may be very significant; 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 Montréal and Lake St. Louis areas. Higher outflows from Lake Ontario prior to, or shortly after, this period help 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.