Effects of Regulation

Question 15.    Would water levels have been lower without the St. Lawrence Seaway and Moses-Saunders Dam?


No, water levels of both Lake Ontario and the lower St. Lawrence River would have been higher in 2017, 2018 and 2019 had the St. Lawrence Seaway and Moses-Saunders Dam never been constructed, and the higher levels would have lasted for a longer duration.


In addition to the construction of Moses-Saunders Dam and the navigation locks, the St. Lawrence Seaway project involved significant dredging in the upper St. Lawrence River.  This makes it physically possible to release higher outflows now than prior to the project. 

As a result, higher regulated outflows are generally released when Lake Ontario is high, prior to and following the spring Ottawa River freshet, which lowers Lake Ontario’s water level.  Moses-Saunders Dam makes it possible to reduce outflows to below what they would have been without the project, which reduces the peak level in the lower St. Lawrence River during the spring Ottawa River freshet.

The outflow conditions that would occur without the St. Lawrence Seaway/Moses-Saunders Dam project can be simulated using a relationship between the observed water levels and flows that occurred prior to the construction of the project (Figure 13).  This relationship is known as the pre-project relation.   The left plot compares the actual weekly mean outflow from 2017 through 2019 to the pre-project outflows calculated weekly for the same inflows or supplies to Lake Ontario.   Similarly, the top right plot compares the weekly mean water level of Lake Ontario from 2017 through 2019 to the simulated pre-project level based on the pre-project outflows, while the bottom right plot compares the actual and pre-project weekly mean levels of Lake St. Louis. 

Regulation reduces the severity of high water impacts on both Lake Ontario and the lower St. Lawrence River.  In 2017 and 2019, it reduced the peak and duration of flooding; in 2018, it helped prevent flooding altogether.

As shown, the project and regulation allowed higher outflows than the natural, pre-project river channel before and after the spring.  In 2019, this reduced the peak Lake Ontario level by approximately 35 cm (13.8 inches) and hastened the decline afterwards.  It also allowed outflows to be temporarily reduced during the critical flooding downstream at Lake St. Louis when the Ottawa River flows were at record-highs, reducing the peak level at Lake St. Louis by over 50 cm (1.6 ft).

On its own, regulation cannot prevent flooding during periods of extreme wet weather and high inflows.  However, the project and regulation do reduce the severity of high water impacts, on both Lake Ontario and the lower St. Lawrence River.  During the extremely wet conditions and high inflows in 2017 and 2019, it reduced the peak and duration of flooding that occurred.  In 2018, under more moderately high inflow conditions, the project and regulation were able to help prevent flooding altogether.

Lake Ontario Outflows, Levels and Lake St. Louis Levels
Figure 13: Actual (observed) versus pre-project (simulated) conditions from 2017 through 2019. Channel excavations during construction of the project allowed actual Lake Ontario outflows (left) to be higher before and after spring. This resulted in lower Lake Ontario water levels (top right) in general, and lower Lake St. Louis water levels (bottom right) during the spring of all three years, including 2018 when flooding was prevented.

Question 16.    Did Plan 2014 cause the high water levels?


No, the high water levels were caused by wet weather and record-high inflows from Lake Erie and the Ottawa River. 


The record-high water levels in 2019 followed persistently above-average precipitation across the Great Lakes basin that lasted several months, and which resulted in record-high inflows from Lake Erie, as well as the record-high flows from the Ottawa River system.  These high inflows dictated how outflows from Lake Ontario had to be managed in 2019, and have dictated outflows in general since as far back as 2017.

During the summer and fall of 2018, outflows largely followed the Plan 2014 maximum L-limit. The maximum L-limit sets outflows at the highest rates possible in consideration of the level of Lake Ontario and the impacts that these high flows have on the upper St. Lawrence River, including increased current velocities and low levels on Lake St. Lawrence, both of which affect commercial navigation and recreational boating.  The Plan 2014 maximum L-limit is based on a similar limit within the previous regulation plan, Plan 1958-D.  However, the Plan 2014 L-limit allows for higher outflows than the Plan 1958-D version, as it better reflects modern ship capabilities and commercial navigation infrastructure, as well as knowledge gained through nearly 60 years of operations under the previous regulation plan.

In November and December 2018, as conditions turned wet, water levels of Lake Ontario began to rise.  As they did, outflows were also increased, again according to the maximum safe flow for navigation, which increases as levels of Lake Ontario rise.  This generally continued until ice formation began.  During this two-month period, outflows averaged 8120 m3/s (286,800 cfs), the 7th highest November-December average on record (1900-present) and equivalent to removing 2.19 m (7.19 ft) of water from Lake Ontario during this time.  Nonetheless, total inflows averaged 8840 m3/s (312,200 cfs) over the same time, the 4th highest on record for November-December and equivalent to adding 2.39 m (7.84 ft), which accounts for the water level rise during this period despite the high outflows.

In the second week of January 2019, cold temperatures caused ice to begin forming in the critical areas of the St. Lawrence River.  As has been done regularly since regulation began in 1960, outflows were temporarily reduced as ice formed on the St. Lawrence River to prevent ice jams that could have severely restricted flows and resulted in immediate localized flooding.  These operations are done according to the I-limit of Plan 2014, which prescribes the maximum flow for safe ice formation.  This rule is also based on similar rules and operational experience gained from decades operating under Plan 1958-D.  This past winter, unlike the winter of 2017, the ice cover formed relatively rapidly starting on 11 January and a stable ice cover was largely established a couple of weeks later (Figure 14).  Outflows were generally increased thereafter, again according to the maximum safe flow possible to ensure safe and stable ice conditions continued. 

Lake St. Lawrence ice coverage, 2 February 2019 (satellite imagery:  USGS Landsat)
Figure 14: Lake St. Lawrence ice coverage, 2 February 2019 (satellite imagery: USGS Landsat)


Ice cover on the St. Lawrence River reduces the cross-sectional area and increases the roughness of the channel, which then also causes water levels immediately upstream of Moses-Saunders Dam on Lake St. Lawrence to drop.  From February and continuing into March of 2019, outflows were increased to the maximum flow that could be released from Lake Ontario, while still maintaining minimum water levels on Lake St. Lawrence to protect municipal and domestic water intakes.  These minimum levels also help ensure that the ice cover remains stable.

Despite the necessary reductions to allow for ice formation and to maintain minimum levels on Lake St. Lawrence, outflows from Lake Ontario were very high in comparison to outflows that have been released historically during this period.  The total amount of water released during the three winter months from December 2018 to February 2019 was the 4th highest on record, having only been exceeded in the winters of 1986-87, 1996-97 and recently in 2017-2018. 

Outflows in March remained high and near or at the maximum outflow that would maintain minimum Lake St. Lawrence levels.  Cold weather also caused ice conditions to persist somewhat later than normal.  On 26 March, the St. Lawrence Seaway opened for the season, and with Lake Ontario still high, outflows began to be set according to the maximum safe flow for navigation.  The outflow in March was the 6th highest on record, and despite well-above average inflows from Lake Erie, Lake Ontario only rose 2 cm (0.8 inches), the 11th smallest rise recorded during the month of March since 1900.

Cool weather and high outflows continued into mid-April.  The cool weather maintained a very deep, dense snowpack in the Ottawa River basin.  Starting on 14 April, milder temperatures and heavy rainfall began and persisted for several days.  This rain combined with a rapid snowmelt, caused Ottawa River flows to increase to record-rates, exceeding the previous record-high set in 2017.  With rising levels on Lake St. Louis due to the Ottawa River outflow, the Plan 2014 F-limit began to apply to balance high water upstream on Lake Ontario and the upper St. Lawrence River with high water downstream on Lake St. Louis and the lower St. Lawrence River.

The outflows during most of the summer/fall of 2018 and the first five months of 2019 were set according to rules of Plan 2014 – namely, the “I” (ice) limit, “L”-limit and “F” (flood) limit (a description of these limits is also available online in the Board’s FAQs).  These are  all maximum flow limits, designed on the basis of how the Board had operated during similar conditions in the past when it often had to deviate from Plan 1958-D to achieve similar results.  The maximum flow limits only apply when Lake Ontario outflows prescribed by Plan 2014 are high, and their intent is to limit outflows from being so high as to cause immediate impacts to interests within the St. Lawrence River system.

At the start of May 2019, water levels exceeded the Criterion H14 high triggers, giving the Board authority to deviate from the rules of Plan 2014.  Outflows were increased throughout the month of May as the Ottawa River generally subsided (Figure 15), in order to continue balancing ongoing high water conditions in the lower St. Lawrence River with increasing water levels upstream on Lake Ontario and the upper St. Lawrence River.  Beginning in June, the Board deviated from the rules of Plan 2014, and released higher outflows to provide relief to Lake Ontario shoreline property owners.   Starting on 10 June, as flooding conditions subsided downstream, outflows were increased above those prescribed by Plan 2014, up to the maximum possible without stopping commercial navigation on the St. Lawrence Seaway.  At this time, outflows matched the highest flows ever previously released on a sustained basis, which first occurred during the summer of 2017, and these unprecedented outflows were maintained from mid-June into mid-August 2019.  Outflows were then gradually reduced as Lake Ontario levels declined, but remained 200 m3/s (7100 cfs) above the normal maximum L-limit flow for safe navigation (Figure 16).  This higher rate of outflow enabled continued lowering of Lake Ontario, while also maintaining safe conditions in the St. Lawrence River.

Lake Ontario levels and Lake St. Louis levels
Figure 15: Lake Ontario outflow reductions in spring 2019 compared to timing of peak Ottawa River flows, Lake Ontario levels and Lake St. Louis levels


In summary, the Board managed outflows during the unusual and extreme weather conditions from November 2018 through late-May 2019 according to Plan 2014 rules that were based on Board operations under the previous regulation plan, Plan 1958-D.  During this entire period, water supplies coming into Lake Ontario were consistently high, reaching record-breaking levels in May, and neither regulation plan would have been able to take significantly more water off Lake Ontario quickly enough to make a meaningful difference in water levels and prevent the flooding in 2019.

Plan 2014 Maximum L-Limit Flow Graph
Figure 16: The Plan 2014 L-limit prescribes the maximum outflow that can be released for a given Lake Ontario level, while still permitting safe navigation in the St. Lawrence River. This maximum outflow declines as water levels decline because as they do this reduces the capacity and increases current velocities in the St. Lawrence River. However, the Plan 2014 L-limit prescribes higher outflows than both pre-project conditions and the Plan 1958-D L-limit, and in 2019 outflows above the L-limit were released for an extended period as the Board deviated from Plan 2014 and tried to remove additional water from Lake Ontario.

Question 17.    Did Plan 2014 hold water back to intentionally raise the level Lake Ontario?


No, water levels rose because of high inflows, which are uncontrolled, even while Plan 2014 outflows were well above average and at maximum flow limits throughout much of 2018 and 2019.


The uncontrolled, natural factors that directly influenced Lake Ontario water levels in 2018 and 2019 were also the primary factors in determining – and often constraining – outflows during this time, including:

  • wet weather in late-fall and winter;
  • ice conditions in the St. Lawrence River in winter;
  • high and eventually record-high inflows from Lake Erie;
  • deep, dense snowpack and late snowmelt coinciding with major rainfall resulting in record-flows from the Ottawa River basin; and,
  • wet spring conditions around Lake Ontario itself. 

It was these natural factors that at times limited how much outflow could be released under Plan 2014, not the rules themselves.  These natural factors would have had the same impact on the system and similar reductions in outflow would have needed to be considered under any regulation plan, including the previous Plan 1958-D. 

In fact, despite these natural constraints, total releases from Lake Ontario under Plan 2014 were well above average during much of 2018 and 2019.  During the summer and fall of 2018, a combination of high Plan 2014 outflows and near-average levels of Lake Ontario resulted in very low (and at times record-low) levels of Lake St. Lawrence, limiting further outflow increases during this period.  Following that, and notwithstanding the reductions needed to establish a stable ice cover, high outflows continued to be prescribed by Plan 2014 during most of the winter, again resulting in very low levels on Lake St. Lawrence.

Outflows were temporarily reduced according to Plan 2014 during the record Ottawa River freshet in spring.  While these reductions are built into the F-limit rule of Plan 2014, similar actions have been taken in the past under the previous regulation plan, and regulation of outflows under any plan would have to be done in consideration of the exceptional spring conditions and the resulting impacts, both upstream and downstream.   Following the spring, outflows were increased to record-setting rates as downstream flooding subsided in early-June.  As described in a previous answer, without the project the levels of Lake Ontario would have been even higher in 2019.

Figure 17: St. Lawrence River at Lake St. Louis, Maple Grove, Quebec, 2 June 2019 (source: International Joint Commission)
Figure 17: St. Lawrence River at Lake St. Louis, Maple Grove, Quebec, 2 June 2019 (source: International Joint Commission)


Figure 18: Lake Ontario at Greece, New York, 28 May 2019 (source: International Lake Ontario – St. Lawrence River Board)
Figure 18: Lake Ontario at Greece, New York, 28 May 2019 (source: International Lake Ontario – St. Lawrence River Board)

Question 18.    Did operation of dams on the Ottawa River system help to reduce flows to the St. Lawrence River?


Yes. While the IJC has no authority over the dams in the Ottawa River system, as was the case in 2017, those dams were operated to reduce high flows and flooding all along the Ottawa River, and this helped reduce peak flows into the St. Lawrence River near Montreal as well.  


In response to the extreme flooding on the Ottawa River system, every measure was taken to reduce discharges from upstream reservoirs.  Further information on operations in the Ottawa River system are available on the Ottawa River Regulation and Planning Board website and a video presentation summarizing the 2019 high water event is also available.

Flood reservoirs in the northern, regulated parts of the Ottawa River basin were lowered during winter to create space to allow much of the spring runoff to accumulate in the reservoirs. The storage space allowed for reduced river flows during the spring to reduce peak flood levels throughout the Ottawa River.  These actions also reduced the peak flows entering the St. Lawrence River and this helps mitigate flood impacts in the Montreal area as well.  For example, at the peak of the Ottawa River flood on 30 April 2019, the combined flow reduction due to the reservoir storage amounted to approximately 5000 m3/s (176,600 cfs) in reduced discharge from Carillon Dam, the most downstream dam on the Ottawa River.  This equates to more than 50 cm (1.6 ft) at Lake St. Louis on the St. Lawrence River and is in addition to the effects that Lake Ontario regulation had in reducing the peak levels here.  During flood events, the safety and security of riparian residents and the integrity of water retention structures take priority over hydropower production.

While flow reductions were significant, there are limitations to using the dams to reduce flows on the Ottawa River, particularly under the extreme conditions experienced in 2019. Storage capacity in the Ottawa River basin is small compared to the total volume of the annual spring freshet (i.e. the surge that occurs in the spring when rains combine with snowmelt). Total runoff from the 2019 spring freshet was nearly four times the total storage volume of the reservoirs in the basin. In addition, approximately 60 percent of the drainage area of the Ottawa River basin is uncontrolled and has no significant reservoir storage capacity. The extreme rainfall received in late-April and early-May 2019 was widespread, including significant totals in the northern part of the Ottawa River basin where there was heavy snow waiting to melt, and also over the uncontrolled southern portion of the basin at the downstream end of the system, closest to the St. Lawrence River. The physical geography of this southern area does not allow further storage of flood waters – in fact, this was clearly illustrated in 2017 and 2019 by the extensive and severe flooding that occurred along the lower Ottawa River during the record-flows in late-April and early-May. 

Question 19.    Would the Board have had more flexibility to release water if Plan 1958-D had been in place leading up to the record-high levels of 2019?


Likely not, as outflows from Plan 2014 were already near or at the maximum that could be released from Lake Ontario without causing additional impacts on interests throughout the system.


It is impossible to know exactly how the Board would have responded under Plan 1958-D in the lead-up to the spring flooding.  Such decisions are based on uncertain information and with considerable subjectivity. 

However, during extremely wet conditions, the constraints imposed by the capacity of the system are the same, and outflows with the new plan are very similar to those that would have occurred operating with the old plan.  This is because the Plan 2014 rules that apply during such wet conditions were designed to reflect the Plan 1958-D rules plus what was learned through years of experience with deviations from Plan 1958-D during similarly wet conditions in the past.  Under Plan 1958-D, the Board needed to deviate frequently from the outflow rules to address conditions (the plan plus deviations is known as 1958-DD).  Deviations from Plan 1958-D rules were often needed to address winter ice conditions, adjust for the effects of high or low Ottawa River flows on the lower St. Lawrence River, and to maximize outflows to moderate Lake Ontario flooding while maintaining safe conditions for navigation.  These conditions had to be addressed by deviations from Plan 1958-D, but are now included and considered directly in the outflow rules for Plan 2014.

Natural factors – including record-high inflows from Lake Erie and from the Ottawa River system – would have had the same impact on the system, and similar reductions in outflow would have needed to be considered under any regulation plan, including the previous Plan 1958-D

For example, had the Board been operating under Plan 1958-D prior to the spring flooding in 2019, the Board would have certainly considered the potential increased risk that high inflows from Lake Erie throughout 2018 would have posed to Lake Ontario.  However, high Lake Erie flows have historically not proven to be a strong indicator of spring flooding during the following year.  Furthermore, Lake Ontario had reached seasonal average levels by the summer, and remained near average through the fall of 2018.  Plan 2014 was nonetheless prescribing maximum L-limit outflows throughout this time, largely in response to the continuing high inflows from Lake Erie, resulting in extremely low levels on Lake St. Lawrence, immediately upstream of Moses-Saunders Dam.

Given this, it is possible that the Board may have operated in such a way that it would have caused slightly higher levels on Lake Ontario and the St. Lawrence River in the latter half of 2018 and possibly continuing into 2019.  Under Plan 1958-D the Board may have reduced outflows below the maximum safe flow for navigation during the summer of 2018 in order to raise levels of Lake St. Lawrence and thereby providing some relief to those having to cope with the record-low levels there at the time.  The Board had done this under similar circumstances in the past while operating under Plan 1958-D.

In January 2019, winter weather conditions would have dictated the same outflow reductions under both plans to ensure safe ice formation and lower the risk of ice jams in the St. Lawrence River.  Stable ice conditions allowed high outflows in February and March 2019, but also again caused low water levels upstream of the dam, which increased risk to municipal water intakes (this risk would have been considered under both plans).  As ice melted in March, and prior to the Seaway opening later that month, outflows remained very high (just slightly below maximum values).  Given that forecasts at the start of March 2019 did not suggest the extreme weather to come, it is unlikely that the Board would have deviated from Plan 1958-D to increase the outflow significantly; had the Board done so, this would have reduced Lake Ontario water levels by at most 5 cm (less than 2 inches). When the Seaway opened in late-March, Plan 2014 again set outflows at maximum safe rates for navigation, as would have been done under Plan 1958-DD.  During April and May, the record-breaking Ottawa River flows and severe flooding downstream of the dam would have limited outflows from Lake Ontario under both plans.

In summary, additional deviation authority may have resulted in small differences in outflows prior to the spring of 2019, but this would have made little difference in the water level of Lake Ontario and the St. Lawrence River by spring when record-high inflows began, and the differences may have made flood conditions worse, not better.

Question 20.    Why not draw Lake Ontario down each fall so that there is sufficient storage to prevent flooding in the spring?


The outcome would have immediate consequences on other interests and will not reliably prevent future flooding impacts on Lake Ontario. 


Plan 2014 does attempt to draw down Lake Ontario levels when water supplies are high, as they have been in the fall of 2017, 2018 and again in 2019.  This helped to successfully prevent flooding in 2018.  However, the physical capacity simply does not exist to prevent flooding in years when winter and spring water supplies upstream and downstream are as extreme as those experienced in 2017 and again in 2019.

Previous studies have shown that it is not possible to prevent all flooding on Lake Ontario, even if this was the only objective of the regulation plan (e.g., see final report by the International Lake Ontario – St. Lawrence River Study Board, pg 33).  Furthermore, the impacts that can occur during low water years must be considered. For example, if there is a drought in the following spring, drawing down Lake Ontario as much as possible each fall would result in significant impacts to other interests.  Extreme fall drawdowns are also known to have detrimental impacts to environmental restoration interests as well.