Lake Ontario Outflow Changes
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The regulation plan (Plan 2014) specifies a weekly average Lake Ontario outflow from Saturday through the following Friday. Throughout the year, the Lake Ontario outflow, which is managed at the Moses-Saunders Dam, may be modified more frequently depending on the conditions in Lake Ontario and the St. Lawrence River. There are two different types of outflow modifications: 

  1. Operational Adjustments, which do not have to be offset at a later date, and
  2. Deviations, which are classified as "minor" or "major" and generally do have to be offset at a later date.

Further details are available in the Directive on Operational Adjustments, Deviations and Extreme Conditions (https://ijc.org/en/loslrb/who/directives/deviations)

 

Access Flow Change Records:

A tabular summary of flow changes is available in an expandable section at the bottom of this page.

Similarly, records of flow changes from previous years are available in an expandable section at the bottom of this page.

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Notices

The outflow target for the week of January 11 through January 17 is the applicable Rule Curve flow of 6,800 m3/s. However, the Lake Ontario outflow has been reduced to 6,260 m3/s to promote the formation of an ice cover in the Beauharnois Canal. 

During the winter months, the outflow will continue to be operationally adjusted in accordance with the Plan 2014 I Limit, as needed, to promote the formation and stability of ice covers in the Beauharnois Canal and on Lake St. Lawrence.

For more information,

The outflow target for the week of January 4 through January 10 is the applicable Rule Curve flow of 6,820 m3/s.

The outflow target for the week of December 28, 2024 through January 3, 2025 is the applicable Rule Curve flow of 6,780 m3/s.

The outflow target for the week of December 21 through December 27 is the applicable Rule Curve flow of 6,800 m3/s.

The outflow target for the week of December 14 through December 20 is the applicable Rule Curve flow of 6,740 m3/s.

The Rule Curve flow for the week of December 7 to December 13 is 6,690 m3/s. However, the outflow will be set to the Plan 2014 prescribed L Limit flow of 6,560 m3/s. The L Limit rules define maximum Lake Ontario outflows that ensure safe operating conditions (i.e. adequate water levels and safe currents) for commercial navigation and other vessels in the St. Lawrence River. 

From Table B3 of the Plan 2014 Compendium: 

Lake Ontario level (m, IGLD 1985)L limit flow (m3/s)
> 74.34 and ≤ 74.54

6,111 + 9,100*(Lake Ontario level - 74.34)

Note: The value 6,111 is first rounded to 6,110

For a Lake Ontario level of 74.39 m, the L Limit flow is computed as: 6,110 + 9,100*(74.39 - 74.34) = 6,565 m3/s, which rounds to 6,560 m3/s. 

For more information on the L Limit, please review FAQ 4.8.5 “What is the L Limit?”

The Rule Curve flow for the week of November 30 to December 6 is 6,780 m3/s. However, the outflow will be set to the Plan 2014 prescribed L Limit flow of 6,470 m3/s. The L Limit rules define maximum Lake Ontario outflows that ensure safe operating conditions (i.e. adequate water levels and safe currents) for commercial navigation and other vessels in the St. Lawrence River. 

From Table B3 of the Plan 2014 Compendium: 

Lake Ontario level (m, IGLD 1985)L limit flow (m3/s)
> 74.34 and ≤ 74.54

6,111 + 9,100*(Lake Ontario level - 74.34)

Note: The value 6,111 is first rounded to 6,110

For a Lake Ontario level of 74.38 m, the L Limit flow is computed as: 6,110 + 9,100*(74.38 - 74.34) = 6,474 m3/s, which rounds to 6,470 m3/s. 

For more information on the L Limit, please review FAQ 4.8.5 “What is the L Limit?”

The Rule Curve flow for the week of November 23 through 29 is 6,580 m3/s. However, the outflow will be set to the Plan 2014 prescribed L Limit flow of 6,470 m3/s. The L Limit rules define maximum Lake Ontario outflows that ensure safe operating conditions (i.e. adequate water levels and safe currents) for commercial navigation and other vessels in the St. Lawrence River. 

From Table B3 of the Plan 2014 Compendium: 

Lake Ontario level (m, IGLD 1985)L limit flow (m3/s)
> 74.34 and ≤ 74.54

6,111 + 9,100*(Lake Ontario level - 74.34)

Note: The value 6,111 is first rounded to 6,110

For a Lake Ontario level of 74.38 m, the L Limit flow is computed as: 6,110 + 9,100*(74.38 - 74.34) = 6,474 m3/s, which rounds to 6,470 m3/s. 

For more information on the L Limit, please review FAQ 4.8.5 “What is the L Limit?”

The outflow target for the week of November 16 through November 22 is the applicable Rule Curve flow of 6,820 m3/s.

The outflow target for the week of November 9 through November 15 is the applicable Rule Curve flow of 6,870 m3/s.

The outflow target for the week of November 2 through November 8 is the applicable Rule Curve flow of 6,920 m3/s.

The outflow target for the week of October 26 through November 1 is 7,060 m³/s which is 80 m³/s higher than the Rule Curve flow prescribed by Plan 2014. A slightly higher outflow will be released to offset the minor deviations that were conducted on September 4, September 24 and the week of September 28 through October 4. The higher flow, relative to Plan 2014, will accelerate the seasonal decline of water levels upstream of the Moses-Saunders Dam by approximately 3-4 cm. As water levels throughout the system continue their gradual seasonal decline, the Board encourages recreational boaters to monitor the water level forecasts to determine the optimal time to remove their vessels and equipment for the season.

The outflow target for the week of October 19 through 25 is 7,090 m³/s which is 90 m³/s higher than the Rule Curve flow prescribed by Plan 2014. For the next two weeks, from October 19 through November 1, slightly higher outflows from Lake Ontario will be released to offset the minor deviations that were conducted on September 4, September 24 and the week of September 28 through October 4. The higher flow, relative to Plan 2014, will accelerate the seasonal decline of water levels upstream of the Moses-Saunders Dam by approximately 3-4 cm. As water levels throughout the system continue their gradual seasonal decline, the Board encourages recreational boaters to monitor the water level forecasts to determine the optimal time to remove their vessels and equipment for the season.

The outflow target for the week of October 12 through October 18 is 7,220 m³/s, as prescribed by Plan 2014.  The Board has authorized a minor deviation to adjust flows through October 18 to target a minimum weekly mean Lake St. Lawrence water level of 73.1 m (239.8 ft). With the plan-prescribed flow of 7,220 m³/s, the weekly mean Lake St. Lawrence water level is expected to be approximately 73.1 m (239.8 ft). Since the Lake St. Lawrence water level is expected to remain near the target of 73.1 m (239.8 ft), a deviation will not be conducted this week. Note that Lake St. Lawrence water levels may fluctuate depending on wind conditions and peaking operations (variations in flow during the day to meet periods of peak energy demand) at the Moses-Saunders Dam. For more information, please visit The International Lake Ontario- St. Lawrence River Board will deviate from Plan 2014 flows this fall | International Joint Commission (ijc.org)

The outflow target for the week of October 5 through October 11 is 7,360 m³/s, as prescribed by Plan 2014.  The Board has authorized a minor deviation to adjust flows through October 18 to target a minimum weekly mean Lake St. Lawrence water level of 73.1 m (239.8 ft). With the plan-prescribed flow of 7,360 m³/s, the weekly mean Lake St. Lawrence water level is expected to be approximately 73.1 m (239.8 ft). Since the Lake St. Lawrence water level is expected to remain near the target of 73.1 m (239.8 ft), a deviation will not be conducted this week. Note that Lake St. Lawrence water levels may fluctuate depending on wind conditions and peaking operations (variations in flow during the day to meet periods of peak energy demand) at the Moses-Saunders Dam. For more information, please visit The International Lake Ontario- St. Lawrence River Board will deviate from Plan 2014 flows this fall | International Joint Commission (ijc.org)

The outflow target for the week of September 28 through October 4 is 7,420 m³/s, which is 60 m³/s below the Rule Curve flow prescribed by Plan 2014.  The Board has authorized a minor deviation to adjust flows through October 18 to target a minimum weekly mean Lake St. Lawrence water level of 73.1 m (239.8 ft). Note that Lake St. Lawrence water levels may fluctuate depending on wind conditions and peaking operations (variations in flow during the day to meet periods of peak energy demand) at the Moses-Saunders Dam. For more information, please visit The International Lake Ontario- St. Lawrence River Board will deviate from Plan 2014 flows this fall | International Joint Commission (ijc.org)

For a 12-hour period on September 24, the outflow will be temporarily reduced to 7,100 m³/s. The Board has authorized this minor deviation to accommodate inspections of Hydro-Québec facilities. For more information, please visit: https://ijc.org/en/loslrb/international-lake-ontario-st-lawrence-river-board-approves-minor-deviation-request-hydro

The outflow target for the week of September 21 through September 27 is 7,510 m³/s, as prescribed by Plan 2014.  The Board has authorized a minor deviation to adjust flows through October 18 to target a minimum weekly mean Lake St. Lawrence water level of 73.1 m (239.8 ft). With the plan-prescribed flow of 7,510 m³/s, the weekly mean Lake St. Lawrence water level is expected to be approximately 73.1 m (239.8 ft). Since the Lake St. Lawrence water level is expected to remain near the target of 73.1 m (239.8 ft), a deviation will not be conducted this week. Note that Lake St. Lawrence water levels may fluctuate depending on wind conditions and peaking operations (variations in flow during the day to meet periods of peak energy demand) at the Moses-Saunders Dam. For more information, please visit The International Lake Ontario- St. Lawrence River Board will deviate from Plan 2014 flows this fall | International Joint Commission (ijc.org)

The outflow target for the week of September 14 through September 20 is 7,600 m³/s, as prescribed by Plan 2014.  The Board has authorized a minor deviation to adjust flows through October 18 to target a minimum weekly mean Lake St. Lawrence water level of 73.1 m (239.8 ft). With the plan-prescribed flow of 7,600 m³/s, the weekly mean Lake St. Lawrence water level is expected to be approximately 73.2 m (240.2 ft). Since the Lake St. Lawrence water level is expected to remain above the target of 73.1 m (239.8 ft), a deviation will not be conducted this week. Note that Lake St. Lawrence water levels may fluctuate depending on wind conditions and peaking operations (variations in flow during the day to meet periods of peak energy demand) at the Moses-Saunders Dam. For more information, please visit The International Lake Ontario- St. Lawrence River Board will deviate from Plan 2014 flows this fall | International Joint Commission (ijc.org)

The outflow target for the week of September 7 through September 13 is 7,580 m3/s, as prescribed by Plan 2014.  The Board has authorized a minor deviation to adjust flows through October 18 to target a minimum weekly mean Lake St. Lawrence water level of 73.1 m (239.8 ft). With the plan-prescribed flow of 7,580 m3/s, the weekly mean Lake St. Lawrence water level is expected to be approximately 73.2 m (240.2 ft). Since the Lake St. Lawrence water level is expected to remain above the target of 73.1 m (239.8 ft), a deviation will not be conducted this week. Note that Lake St. Lawrence water levels may fluctuate depending on wind conditions and peaking operations (variations in flow during the day to meet periods of peak energy demand) at the Moses-Saunders Dam. For more information, please visit The International Lake Ontario- St. Lawrence River Board will deviate from Plan 2014 flows this fall | International Joint Commission (ijc.org)

For a 12-hour period on September 4, the outflow will be temporarily reduced to 6,720 m3/s. The Board has authorized this minor deviation to ensure safe conditions for in-water inspections of Hydro-Quebec facilities. For more information, please visit: The International Lake Ontario- St. Lawrence River Board will deviate from Plan 2014 flows this fall | International Joint Commission (ijc.org)

The outflow target for the week of August 31 through September 6 is 7,820 m3/s, as prescribed by Plan 2014.  The Board has authorized a minor deviation to adjust flows through October 18 to target a minimum weekly mean Lake St. Lawrence water level of 73.1 m (239.8 ft). With the plan-prescribed flow of 7,820 m3/s, the weekly mean Lake St. Lawrence water level is expected to be approximately 73.2 m (240.2 ft). Since the Lake St. Lawrence water level is expected to remain above the target of 73.1 m (239.8 ft), a deviation will not be conducted this week. As of August 29, the Lake St. Lawrence water level is slightly below 73.1 m (239.8 ft) because of recent northeasterly winds. The Lake St. Lawrence water level is expected to rise as the wind shifts direction heading into the weekend. For more information, please visit The International Lake Ontario- St. Lawrence River Board will deviate from Plan 2014 flows this fall | International Joint Commission (ijc.org)

The outflow target for the week of August 24 through August 30 is the applicable Rule Curve flow of 7,820 m3/s.

The outflow target for the week of August 17 through August 23 is the applicable Rule Curve flow of 7,830 m3/s.

The outflow target for the week of August 10 through August 16 is the applicable Rule Curve flow of 7,820 m3/s.

The outflow target for the week of August 3 through August 9 is the applicable Rule Curve flow of 7,950 m3/s.

The outflow target for the week of July 27 through August 2 is the applicable Rule Curve flow of 8,120 m3/s.

The outflow target for the week of July 20 through July 26 is the applicable Rule Curve flow of 8,230 m3/s.

The outflow target for the week of July 13 through July 19 is the applicable Rule Curve flow of 7,980 m3/s.

The outflow target for the week of July 6 through July 12 is the applicable Rule Curve flow of 7,890 m3/s.

The outflow target for the week of June 29 through July 5 is the applicable Rule Curve flow of 7,880 m3/s.

The outflow target for the week of June 22 through June 28 is the applicable Rule Curve flow of 7,800 m3/s.

The Board has approved a minor deviation consisting of a 10-hour period of reduced flows on June 19, 2024, to ensure safe conditions for in-water inspections of Long Sault Dam. After the 10-hour period, the total flow will increase to 7980 m3/s, which is 100 m3/s above the Plan 2014 Rule Curve flow of 7880 m3/s. By the end of the week, the minor deviation will be fully offset and there will be no cumulative impact on the water level of Lake Ontario. Outflow will return to the applicable plan-prescribed Rule Curve flow on June 22.

The outflow target for the week of June 15 through June 21 is the applicable Rule Curve flow of 7,880 m3/s.

The outflow target for the week of June 8 through June 14 is the applicable Rule Curve flow of 7,880 m³/s.

The outflow target for the week of June 1 through June 7 is the applicable Rule Curve flow of 7,960 m³/s.

The outflow target for the week of May 25 through May 31 is the applicable Rule Curve flow of 7,960 m³/s. 

The outflow target for the week of May 18 through May 24 is the applicable Rule Curve flow of 7,980 m3/s.

The outflow target for the week of May 11 through May 17 is the applicable Rule Curve flow of 8,020 m3/s.

The outflow target for the week of May 4 through May 10 is the applicable Rule Curve flow of 8,070 m3/s.

The outflow target for the week of April 20 through April 26 is the applicable Rule Curve flow of 8,120 m³/s. 

The outflow target for the week of April 6 through April 12 is the applicable Rule Curve flow of 7,760 m³/s.

The outflow target for the week of March 9 through March 15 is the applicable Rule Curve flow of 7,700 m³/s. 

The outflow target for the week of February 10 through February 16 is the applicable Rule Curve flow of 7,820 m3/s.

If ice resumes forming in the Beauharnois Canal, the outflow will be operationally adjusted in accordance with the Plan 2014 I Limit.

The outflow target for the week of February 3 through February 9 is the applicable Rule Curve flow of 7,800 m3/s.

If ice resumes forming in the Beauharnois Canal, the outflow will be operationally adjusted in accordance with the Plan 2014 I Limit. The ice cover that had formed in January has melted. As of today (February 1), there is no significant ice cover in the Beauharnois Canal or on Lake St. Lawrence.

(Updated)

The outflow target for the week of January 20 through January 26 is the applicable Rule Curve flow of 7,560 m3/s.

However, the Lake Ontario outflow has been reduced to promote the formation of an ice cover in the Beauharnois Canal. About 1 km of ice has accumulated in the Beauharnois Canal.

During the winter months, the outflow will continue to be operationally adjusted in accordance with the Plan 2014 I Limit, as needed, to promote the formation and stability of ice covers in the Beauharnois Canal and on Lake St. Lawrence.

For more information,

Tabular Summary of Lake Ontario Outflow Changes
DATEHOUROUTFLOWOUTFLOWNOTES
(YYYY-MM-DD)(HHMM)(m3/s)(cfs) 
2025-01-1212016,260221,100Operational Adjustment for I Limit
2025-01-1112016,000211,900Operational Adjustment for I Limit
2025-01-0918015,700201,300Operational Adjustment for I Limit
2025-01-0913016,260221,100Operational Adjustment for I Limit
2025-01-0400016,820240,800Rule Curve
Outflow change records from previous years

To view outflow change records from previous years, please use the links below:

Flow Changes - 2024

Flow Changes - 2023

Flow Changes - 2022

Flow Changes - 2021

Flow Changes - 2020

Flow Changes - 2019

Flow Changes - 2018

Flow Changes - 2017

Flow Changes - 2016

Flow Changes - 2015

Flow Changes - 2014

Flow Changes - 2013

Flow Changes - 2012

Flow Changes - 2011