Outflows in 2018 & 2019

Question 7.    Why wasn’t more water released in 2018 when Lake Erie levels were high and downstream flooding wasn’t an issue?


A lot of water was released from Lake Ontario in 2018 for exactly this reason, and this, along with near-average water levels of Lake Ontario, contributed to very low levels on Lake St. Lawrence during this time. 


Prior to the record-high water levels in spring 2019, Lake Erie had been relatively high for several years, as had the other Great Lakes.  Outflows from Lake Ontario have reflected this, and in fact, have also consistently been kept very high since the record-flooding of 2017, with the exception of the temporary reductions required during the springs of 2017 and 2019 when flooding was occurring both upstream and downstream. 

In fact, throughout the summer and fall of 2018, while Lake Ontario levels were at or near the long-term-average, the high Lake Ontario outflows resulted in very low, and at times record-low, water levels on Lake St. Lawrence, immediately upstream of Moses-Saunders Dam (Figure 8).  Click here for a video describing the effects of regulation on water levels in Lake St. Lawrence.  Low levels, as well as high current velocities in this area, had impacts on recreational boaters, shoreline property owners and commercial navigation. 

Lake St. Lawrence at Croil Island near Massena, New York, 16 September 2018 (source:  International Lake Ontario – St. Lawrence River Board)
Figure 8: Lake St. Lawrence at Croil Island near Massena, New York, 16 September 2018 (source: International Lake Ontario – St. Lawrence River Board)

Question 8.    Why wasn’t more water released from Lake Ontario during the winter?  Why were outflows reduced during January and February?  


Outflows must be temporarily reduced nearly every winter as ice forms on the St. Lawrence River in order to facilitate the formation of a stable ice cover, which reduces the risk of ice jams and allows higher outflows to be released later on.


As ice forms on the St. Lawrence River, outflows can be temporarily reduced to slow down the river current and reduce the forces acting on the fragile ice cover, allowing a solid ice cover to form and stabilize.  This helps reduce the risk of ice jams and limits frazil ice* growth in the river that can physically block and severely restrict flows.  Severe ice jams can result in immediate localized flooding and an immediate reduction in outflows.  If such ice restrictions last for a long duration, outflow may be reduced for an extended period during the winter, leading to higher Lake Ontario levels heading into spring.

In 2019, ice began forming around mid-January and continued into February.  Once ice had formed and stabilized, outflows were increased relatively rapidly thereafter.  In fact, the amount of water released during the winter (from December 2018 – February 2019, combined) was relatively high historically, as only 3 years have seen more water released during these three months, those being 1987, 1997 and last year, 2018. 

* What is frazil ice?

Frazil ice is composed of loosely consolidated, tiny ice particles that form in flowing water.  These particles can look like irregularly shaped pans of slush at the surface, and extreme cold temperatures can cause frazil to form rapidly in open areas.  In deep, fast moving water, frazil ice can be transported within the water column, where it may collect and adhere to other ice particles or to the riverbed, narrowing the channel cross-section and restricting flows.

Question 9.    Why was more water released in January and February 2018 than during the same months in 2019?


Compared to 2019, ice formed more rapidly in early-January 2018 allowing somewhat higher outflows to be released earlier on in winter, but overall, outflows were very high during January and February in both years.


Extreme and, at times, record-cold temperatures at the end of 2017 and start of January 2018 allowed ice to form more rapidly and earlier in 2018 than it did in 2019, and this allowed outflows to be increased sooner as well.   Regardless, outflows released in January and February in both 2018 and 2019 are historically high, the 2nd and 4th highest on record, respectively (Figure 9).  These high winter flows were possible due to consistently cold temperatures and the solid, stable ice conditions that this promoted.  By comparison, ice formation in 2017 was much more complicated, as unusual temperature fluctuations required that Lake Ontario outflows be almost continuously adjusted to manage highly variable ice conditions in the St. Lawrence River. More information on the difficulties of ice management in 2017 can be found in the Board’s report on 2017 conditions here.

Lake Ontario Historical Outflows: Winter (Dec-Jan-Feb) Average
Figure 9: Lake Ontario mean outflows during winter (Dec-Jan-Feb) from 1900-2019. Outflows during the winter of 2018-2019 were on average the 4th highest ever recorded, despite limitations posed by low Lake St. Lawrence levels in December and ice conditions in January/February.

Question 10.    Why were outflows from Lake Ontario reduced during the spring of 2019?


Outflows from Lake Ontario were reduced to mitigate – but not eliminate – damaging flooding in the lower St. Lawrence River during a record-setting Ottawa River freshet in the spring of 2019.


The Ottawa River enters the St. Lawrence River near Montreal and combines with the flow released from Lake Ontario.  Each spring, milder temperatures, snowmelt and rainfall increase Ottawa River flows, and the timing and magnitude needs to be considered carefully when regulating Lake Ontario outflows to manage water levels on the St. Lawrence River (see also Question 16 and Figure 15).

In 2019, a heavy snow-pack on the Ottawa River basin lasted into mid-April as a result of colder than normal temperatures in early-spring.  Major rains fell over the Ottawa River basin in late-April and continued into May.  These rains combined with the rapid, late snowmelt to result in record-high runoff and flows from the Ottawa River into the St. Lawrence River, and in some places even greater water levels than those seen just two years prior during the record flooding of 2017.

Question 11.    Why were outflows during the spring of 2019 lower than during the springs of 2017 and 2018 and why were they lower for so long?


Outflows from Lake Ontario were, at times, lower in the spring of 2019 because Ottawa River flows were higher (record-high, at their peak) and they lasted for a much longer duration.  


Record-high Ottawa River flows lasting a record-duration in 2019 (Figure 10) were caused by the combination of an unusually deep, dense snowpack and a late melt, which occurred at the same time as exceptionally heavy rains in late-April and early-May.

The record-peak Ottawa River daily flow recorded at Carillon Dam in 2019 was 9217 m3/s (325,500 cfs), higher than the previous record of 9094 m3/s (321,200 cfs) set 8 May 2017, and more than 3000 m3/s (105,900 cfs) above the peak daily flow recorded in 2018 of 5860 m3/s (206,900 cfs).  

Ottawa River at Carillon Dam: Daily Outflow (1963-2019)
Figure 10: Ottawa River daily flows at Carillon Dam. Each line represents an individual calendar year (1963-2019). Record flows lasting a record duration occurred in the spring of 2019, all entering the lower St. Lawrence River near Montreal.


The exceptionally high Ottawa River flows in 2019 also lasted for a much longer duration.  For example, Ottawa River flows were above 8000 m³/s (282,500 cfs) for 21 days in 2019, but only 6 days in 2017, and Ottawa River flows were above the highest daily flow recorded in all of 2018 (5860 m3/s (206,900 cfs) on 10 May) from 19 April to 3 June 2019, a full month and a half. 

Ottawa River flows: highest months recorded since completion of Carillon Dam in 1963
Table 3: Ottawa River flows: highest months recorded since completion of Carillon Dam in 1963


The Ottawa River monthly mean flows also set a new record-high for the month of May 2019, exceeding the previous monthly record in 1974 as well as the second highest on record in 2017 by more than 1000 m3/s (35,300 cfs).  April 2019 was also the 5th highest month on record, and the months of April and May 2019 combined far exceed any previous 2-month period.

Question 12.    How can outflows impact navigation safety?


Outflows were increased and sustained at record-high rates for an extended period in the summer of 2019 to increase the rate of decline on Lake Ontario, but this also increased the current velocity of the St. Lawrence River and increased the risks to safe commercial navigation.


The Board’s priority in 2019 was to reduce the impacts of high water conditions on shoreline riparians, businesses and communities both upstream and downstream in the Lake Ontario – St. Lawrence River system.  In doing so, the Board must consider the degree of relief that can be provided, as well as the consequences to all interests, including navigation.

Starting 10 June, as flooding conditions subsided downstream in the lower St. Lawrence River, outflows were increased above the flows that would have been prescribed by Plan 2014 in order to increase the rate of water level decline on Lake Ontario.  By 13 June, outflows reached the record-high outflow of 10,400 m3/s (367,300 cfs).  This flow rate was first achieved in 2017 and constitutes the highest flow ever released on a sustained basis.  These record-matching outflows were maintained from mid-June through mid-August, even longer than in 2017.

When outflows are increased, this increases the velocity of currents in the St. Lawrence River.  To maintain safe conditions for navigation during the sustained, record-high flows, the Seaway entities imposed speed limits, no passing restrictions and other mitigation measures. 

Releasing higher outflows than those set by the Board during the summer of 2019 would have increased currents in the international section of the St. Lawrence River to an extent that would have effectively forced the stoppage of commercial navigation. This would have further impacted people’s lives and disrupted the economy throughout the Great Lakes region, without providing significant additional relief on Lake Ontario.

Furthermore, the gradual decline of Lake Ontario and upper St. Lawrence River levels through the summer months caused the velocity in the river to gradually increase, even while outflow was maintained at the same record-rate. This presented additional challenges, and eventually, maintaining record-high flows was no longer safe for navigation.  As a result, starting in mid-August, flows were gradually reduced to ensure safe river currents and to allow ship transits to continue.

Question 13.    How effective were the deviation strategies of 2017 and 2019 compared to the strategy employed during the summer of 1993, when outflows were set at alternating rates that temporarily stopped commercial navigation?


The Board’s deviation strategy in 2017 and 2019 resulted in continuously higher outflows released for a longer duration when compared to the strategy in 1993.  Record-sustained outflows averaging 10,400 m3/s (366,900 cfs) were released for 54 days in 2017 and 69 days in 2019.  Compared to 1993, when outflows averaged 10,190 m3/s (359,900 cfs) over just 23 days, the strategies employed in 2017 and 2019 allowed more water to be removed from Lake Ontario, a greater rate of lowering and more rapid relief to Lake Ontario riparians, and with fewer impacts on other stakeholders.


In both 2017 and 2019, Lake Ontario outflows were set higher than those released in 1993 on a weekly basis and were sustained for a longer period, while permitting navigation to continue with the Seaway’s mitigation measures in place. The flow of 10,400 m3/s (366,900 cfs) is the highest sustained outflow on record.  In 2019, this flow of 10,400 m3/s was maintained from 13 June to 21 August, and over those 69 days, this record-outflow removed the equivalent of about 3.17 m (10.4 ft) of water from Lake Ontario. This is more than the total number of days that the same record-outflow was sustained during the summer of 2017, when it lasted from 14 June to 8 August, and removed just over 2.5 m (8.2 ft) of water.

The only time higher outflows have ever been released for any duration was during the high-water event of 1993, when temporary, 24-hour flow increases from 9900 to 10,900 m3/s (349,600 to 384,900 cfs) occurred approximately twice per week from 20 May to 11 June 1993. Over this 23-day period, outflows were increased a total of seven times and the Seaway suspended navigation during each of these 24-hour flow increases. The average outflow over the entire 23 days that this strategy was implemented in 1993 was 10,190 m3/s (359,900 cfs), equivalent to removing 104 cm (41 inches) of water from Lake Ontario, much less than what was achieved in 2019 and in 2017, and with greater impacts on other interests.

Overall, the outflows released during the summers of 2017 and 2019 were record-setting (Figure 11). The higher sustained outflows in 2017 and 2019 were made to lower water levels in consideration of Lake Ontario and upper St. Lawrence River riparian interests primarily, but other interests throughout the Lake Ontario – St. Lawrence River system were also considered.  In addition to commercial navigation, the safety of recreational boaters and other users along the St. Lawrence River would have also been impacted by a strategy involving fluctuating outflows, both in the upper and lower St. Lawrence River.  The higher temporary outflows that this strategy would have entailed would have also increased high-water impacts downstream, including flooding and erosion along the lower St. Lawrence River.  Instead, the strategy in 2017 and 2019 resulted in more predictable conditions for all users, while still ensuring record-high releases and a greater rate of lowering of Lake Ontario than was achieved in 1993, and with fewer impacts on other stakeholders. 

Lake Ontario Historical Outflows: Summer (Jun-Jul-Aug) Average
Figure 11: Lake Ontario mean outflows during summer (Jun-Jul-Aug) from 1900-2019. Outflows during the summer of 2019 were the highest ever recorded, exceeding the previous record set in the summer of 2017.

Question 14.    Why were flows reduced in October for Lake St. Lawrence boat haul-out?


High outflows from Lake Ontario contribute to low levels on Lake St. Lawrence.  Flow reductions were necessary to create enough depth for Lake St. Lawrence boaters to access boat ramps and lifts.


The extremely high outflows, especially after Lake Ontario reached lower levels in August, reduced the level of Lake St. Lawrence just upstream (west) of Moses-Saunders Dam at Cornwall, Ontario and Massena, New York. This left many boats grounded, and many marinas and boaters unable to remove their vessels prior to winter. Reducing the outflow for one weekend in the fall (12-13 October 2019) raised the level of Lake St. Lawrence sufficiently to assist with boat haul-out without any significant impact on the level of Lake Ontario.

Lake St. Lawrence at Long Sault, Ontario, 10 October 2018 (source:  International Lake Ontario – St. Lawrence River Board)
Figure 12: Lake St. Lawrence at Long Sault, Ontario, 10 October 2018 (source: International Lake Ontario – St. Lawrence River Board)