Lake Ontario Outflow Changes

The Rule Curve flow for the week of September 12 through 18 is 7,760 m³/s (274,000 ft³/s). When the level of Lake Ontario is above 74.80 m (245.41 ft) at the beginning of September, Plan 2014 prescribes an increase to the Rule Curve flow. Based on the current level of Lake Ontario (75.20 m or 246.72 ft), a 910 m³/s (32,100 ft³/s) increase would be prescribed for a total flow of 8,670 m³/s (306,200 ft³/s). However, Plan 2014 also includes a rule called the J Limit to prevent rapid velocity and water level changes in the St. Lawrence River.  Based on the current Lake Ontario level, the J Limit restricts the week-to-week increase in flow to 700 m³/s (24,700 ft³/s). Therefore, the Plan 2014 prescribed flow for the week of September 12 through 18 is 8,480 m³/s (299,500 ft³/s), in accordance with the J Limit.

The flow increase to the J Limit will be implemented in two steps of +350 m³/s (12,400 ft³/s) by increasing the flow first to 8,130 m³/s (287,100 ft³/s) this afternoon (Friday, September 11), and then to 8,480 m³/s (299,500 ft³/s) tomorrow afternoon (Saturday, September 12). These incremental flow increases will moderate the transition in flow and the resulting increase in water levels downstream of the Moses-Saunders Dam. Increasing outflows causes Lake St. Lawrence water levels to decline, however, the water levels of Lake St. Lawrence are expected to rise as Ontario Power Generation begins raising the gates of Iroquois Dam.  Iroquois Dam has not been available for recreational boat passage this summer because the gates were closed to mitigate high water levels on Lake St. Lawrence. 

The outflow target for the week of August 22 through August 28 is the applicable Rule Curve flow of 7,780 m³/s (274,700 ft³/s), as prescribed by Plan 2014. 

The gates of Iroquois Dam currently remain partially lowered to prevent high water levels on Lake St. Lawrence. 

The outflow target for the week of August 15 through August 21 is the applicable Rule Curve flow of 7,860 m³/s (277,600 ft³/s), as prescribed by Plan 2014. 

The gates of Iroquois Dam currently remain partially lowered to prevent high water levels on Lake St. Lawrence.

The outflow target for the week of August 8 through August 14 is the applicable Rule Curve flow of 8,020 m³/s (283,200 ft³/s), as prescribed by Plan 2014. 

The gates of Iroquois Dam remain partially lowered to prevent high water levels on Lake St. Lawrence. 

The outflow target for the week of August 1 through August 7 is the applicable Rule Curve flow of 7,860 m³/s (277,600 ft³/s), as prescribed by Plan 2014. 

The gates of Iroquois Dam remain partially lowered to prevent high water levels on Lake St. Lawrence. 

The outflow target for the week of July 25 through July 31 is the applicable Rule Curve flow of 7,900 m³/s (279,000 ft³/s), as prescribed by Plan 2014. 

The gates of Iroquois Dam remain partially lowered to prevent high water levels on Lake St. Lawrence. 

The outflow target for the week of July 18 through July 24 is the applicable Rule Curve flow of 7,820 m³/s (276,200 ft³/s), as prescribed by Plan 2014. 

The gates of Iroquois Dam remain partially lowered to prevent high water levels on Lake St. Lawrence. 

The outflow target for the week of July 11 through July 17 is the applicable Rule Curve flow of 8,000 m³/s (282,500 ft³/s), as prescribed by Plan 2014. 

The gates of Iroquois Dam remain partially lowered to prevent high water levels on Lake St. Lawrence. 

The outflow target for the week of July 4 through July 10 is the applicable Rule Curve flow of 7,980 m³/s (281,800 ft³/s), as prescribed by Plan 2014. 

The gates of Iroquois Dam remain partially lowered to prevent high water levels on Lake St. Lawrence. 

The outflow target for the week of June 27 through July 3 is the applicable Rule Curve flow of 8,080 m³/s (285,300 ft³/s), as prescribed by Plan 2014. 

The gates of Iroquois Dam remain partially lowered to prevent high water levels on Lake St. Lawrence. 

The outflow target for the week of June 20 through June 26 is the applicable Rule Curve flow of 8,000 m³/s (282,500 ft³/s), as prescribed by Plan 2014. 

The gates of Iroquois Dam remain partially lowered to prevent high water levels on Lake St. Lawrence. 

The outflow target for the week of June 13 through June 19 is the applicable Rule Curve flow of 7,930 m³/s (280,000 ft³/s), as prescribed by Plan 2014. 

The gates of Iroquois Dam remain partially lowered to prevent high water levels on Lake St. Lawrence. 

The outflow target for the week of June 6 through June 12 is the applicable Rule Curve flow of 8,040 m³/s (283,900 ft³/s), as prescribed by Plan 2014. 

The gates of Iroquois Dam remain partially lowered to prevent high water levels on Lake St. Lawrence. 

The outflow target for the week of May 30 through June 5 is the applicable Rule Curve flow of 8,100 m³/s (286,000 ft³/s), as prescribed by Plan 2014. 

The gates of Iroquois Dam remain partially lowered to prevent high water levels on Lake St. Lawrence. 

The outflow target for the week of May 23 through 29 is the applicable Rule Curve flow of 8,010 m³/s (282,900 ft³/s), as prescribed by Plan 2014. 

The gates of Iroquois Dam remain partially lowered to prevent high water levels on Lake St. Lawrence. 

The outflow target for the week of May 16 through 22 is the applicable Rule Curve flow of 8,100 m³/s (286,000 ft³/s), as prescribed by Plan 2014. 

The gates of Iroquois Dam remain partially lowered to prevent high water levels on Lake St. Lawrence. 

Conditions in the Ottawa River basin are gradually decreasing, allowing Lake Ontario outflows to increase.

This afternoon, on May 7, the outflow will be increased to 8,020 m³/s, which is Rule Curve flow for the week of May 2 through 8. The Rule Curve flow for the week of May 9 through 15 is 8,060 m³/s. 

The table below shows the operational flow adjustments that have occurred over the past week:

The gates of Iroquois Dam currently remain lowered to prevent high water levels on Lake St. Lawrence. 

As of this morning, the water level of Lake Ontario has reached 75.30 m. The second tier of the F Limit now applies and the outflow will be adjusted to maintain a maximum level of 22.20 m at Lake St. Louis (measured at Pointe Claire). Rain combined with continuing snowmelt in the northern portion of the basin is causing Ottawa River flows to rise again.

The table below shows the operational flow adjustments that have occurred over the past week:

If the level of Lake Ontario rises to 75.37 m, the third F-limit tier will apply, and the outflow will be adjusted to maintain a maximum level of 22.33 m at Lake St. Louis. This may occur in the coming week, if Lake Ontario continues to rise at the current rate of 1 cm per day. 

The Rule Curve flow for the week of May 2 through 8 is 8,020 m³/s. However, actual Lake Ontario outflows are expected to be lower than this, in accordance with the F Limit. The gates of Iroquois Dam remained lowered to prevent high water levels on Lake St. Lawrence. 

Ottawa River flows into the St. Lawrence River are stabilizing and are expected to begin to slowly decline. The rate of decline will depend on the weather. Over the last week, the Lake Ontario outflow was adjusted several times in accordance with the Plan 2014 F Limit. The table below shows the operational flow adjustments over the past week: 

Since the level of Lake Ontario is below 75.3 m, the first F-limit tier applies and the outflow will continue to be adjusted to maintain a maximum level of 22.10 m at Lake St. Louis (measured at Pointe Claire). If the level of Lake Ontario rises to 75.3 m, the second F-limit tier will apply, and the outflow will be adjusted to maintain a maximum level of 22.20 m at Lake St. Louis. To prevent high water levels on Lake St. Lawrence, the gates of Iroquois Dam were lowered on April 17-18 and further lowered on April 21. The Iroquois Dam gates will be raised again as Lake Ontario outflows are gradually increased. 

The Rule Curve flow for the week of April 25 through May 1 is 7,900 m³/s. However, actual Lake Ontario outflows are expected to be lower than this, in accordance with the F Limit.

Ottawa River flows into the St. Lawrence River are increasing rapidly in response to recent rainfall combined with significant snowmelt. As a result, water levels in the lower St. Lawrence River have increased. The Lake Ontario outflow was reduced to 7,200 m³/s today and will continue to be adjusted in accordance with the Plan 2014 F Limit. Since the level of Lake Ontario is below 75.3 m, the first F Limit tier applies and the outflow will be adjusted to maintain a maximum level of 22.10 m at Lake St. Louis (measured at Pointe Claire).

The Rule Curve flow for the week of April 18 through 24 is 7,600 m³/s. However, actual Lake Ontario outflows are expected to be lower than this, in accordance with the F Limit.

The outflow target for the week of April 11 through April 17 is the applicable Rule Curve flow of 7,420 m³/s, as prescribed by Plan 2014.

Depending on weather and water supply conditions during the next several weeks, the Lake Ontario outflow into the St. Lawrence River may need to be adjusted in accordance with the Plan 2014 F Limit. Since the level of Lake Ontario is below 75.3 m, the first F-limit tier applies, and the outflow would be adjusted to maintain a maximum level of 22.10 m at Lake St. Louis (measured at Pointe Claire). 

The outflow target for the week of April 4 through April 10 is the applicable Rule Curve flow of 7,160 m³/s, as prescribed by Plan 2014.

On Friday, March 27, significant ice movement occurred in the Beauharnois Canal and the outflow was temporarily decreased to 6,500 m³/s in accordance with the Plan 2014 I Limit. On Tuesday, March 31, as conditions improved, the outflow was increased to the applicable Rule Curve flow of 6,960 m³/s.

As of the beginning of April, the ice covers on Lake St. Lawrence and in the Beauharnois Canal have mostly cleared. Depending on weather and water supply conditions during the next several weeks, the Lake Ontario outflow into the St. Lawrence River may need to be adjusted in accordance with the Plan 2014 F Limit. Since the level of Lake Ontario is below 75.3 m, the first F-limit tier applies, and the outflow would be adjusted to maintain a maximum level of 22.10 m at Lake St. Louis (measured at Pointe Claire). 

From Table B4 of the Plan 2014 Compendium:

The outflow target for the week of March 28 through April 3 is the applicable Rule Curve flow of 6,960 m³/s, as prescribed by Plan 2014. 

The outflow target for the week of March 21 through March 27 is the applicable Rule Curve flow of 6,860 m³/s, as prescribed by Plan 2014.

The outflow target for the week of March 14 through March 20 is the applicable Rule Curve flow of 6,560 m³/s, as prescribed by Plan 2014.

The outflow target for the week of March 7 through March 13 is the applicable Rule Curve flow of 6,310 m³/s, as prescribed by Plan 2014.

The outflow target for the week of February 28 through March 6 is the applicable Rule Curve flow of 6,390 m³/s, as prescribed by Plan 2014. The ice cover in the Beauharnois Canal and the St. Lawrence River are currently stable. However, if conditions require, the outflow will be operationally adjusted in accordance with the Plan 2014 I Limit to promote the stability of the ice covers.

The outflow target for the week of February 21 through February 27 is the applicable Rule Curve flow of 6,260 m³/s, as prescribed by Plan 2014. The ice cover in the Beauharnois Canal and the St. Lawrence River are currently stable. However, if conditions require, the outflow will be operationally adjusted in accordance with the Plan 2014 I Limit to promote the stability of the ice covers and to maintain the water level of Lake St. Lawrence (measured at Long Sault Dam) near or above 71.8 m to protect municipal water intakes.

The outflow target for the week of February 14 through February 20 is the applicable Rule Curve flow of 6,260 m³/s, as prescribed by Plan 2014. 

The ice cover in the Beauharnois Canal and the St. Lawrence River are currently stable. However, if conditions require, the outflow will be operationally adjusted in accordance with the Plan 2014 I Limit to promote the stability of the ice covers and to maintain the water level of Lake St. Lawrence (measured at Long Sault Dam) near or above 71.8 m to protect municipal water intakes.

The outflow target for the week of February 7 through February 13 is the applicable Rule Curve flow of 6,290 m³/s, as prescribed by Plan 2014. 

The ice cover in the Beauharnois Canal and the St. Lawrence River are currently stable. However, if conditions require, the outflow will be operationally adjusted in accordance with the Plan 2014 I Limit to promote the stability of the ice covers and to maintain the water level of Lake St. Lawrence (measured at Long Sault Dam) near or above 71.8 m to protect municipal water intakes.

The ice cover in the International Section of the St. Lawrence River has progressed upstream of Iroquois Dam and is currently stable, which allowed the Lake Ontario outflow to be gradually increased on January 27 and January 28 in accordance with the Plan 2014 I Limit. The ice cover in the Beauharnois Canal also remains stable.

The outflow target for the week of January 31 through February 6 is the applicable Rule Curve flow of 6,370 m³/s, as prescribed by Plan 2014. If conditions require, the outflow will be operationally adjusted in accordance with the Plan 2014 I Limit to promote the stability of the ice covers and to maintain the water level of Lake St. Lawrence (measured at Long Sault Dam) near or above 71.8 m to protect municipal water intakes.

The outflow target for the week of January 24 through 30 is the applicable Rule Curve flow of 6,490 m³/s, as prescribed by Plan 2014. However, on January 23 the outflow will be reduced to 6,230 m³/s in accordance with the I Limit to promote the progression of the ice cover in the upper St. Lawrence River between Morrisburg, ON and Iroquois, ON. The ice cover in the Beauharnois Canal is currently stable.

In the coming week, 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 the ice covers on Lake St. Lawrence and in the Beauharnois Canal.

The outflow target for the week of January 17 through 23 is the applicable Rule Curve flow of 6,470 m³/s, as prescribed by Plan 2014. If the current flow of 6,240 m³/s remains unchanged today and tomorrow (January 15 and 16), the remaining accumulated minor deviation that was conducted for Lake St. Louis recreational boat haul-out on October 17 will be fully offset by the end of this week (January 16).    

Ice covers remain in the Beauharnois Canal and Lake St. Lawrence but have receded following recent warm temperatures. With the return of colder temperatures, the outflow may need to be operationally adjusted in accordance with the I Limit to promote the stability and potential additional progression of the ice covers. 

Given current conditions in the Ottawa River and the lower St. Lawrence River, operational adjustments in accordance with the M Limit are not expected to be required in the coming weeks. The water level of Lake St. Louis (measured at Pointe Claire) is approximately 20 cm above the applicable M-Limit tier of 20.64 m. 

The outflow target for the week of January 10 through 16 is 6,240 m³/s, which is 60 m³/s below the Rule Curve flow prescribed by Plan 2014. A slightly lower outflow will be released to offset the minor deviation that was conducted for Lake St. Louis recreational boat haul-out on October 17-18, 2025.  

However, if conditions require, the outflow will be operationally adjusted in accordance with the I Limit and/or the M Limit. 

• There are partial ice covers in Beauharnois Canal and on Lake St. Lawrence upstream of the Moses-Saunders Dam. If needed, the outflow will be operationally adjusted in accordance with the Plan 2014 I Limit, to promote the formation and stability of the ice covers.
• Since the level of Lake Ontario is above 74.2 m, the first tier of the M Limit applies, and if needed, the outflow would be adjusted to maintain a minimum level of 20.64 m at Lake St. Louis (measured at Pointe Claire).

The Lake Ontario outflow was operationally adjusted several times between December 26 and December 31 in accordance with the Plan 2014 M Limit. On January 1, 2026, the outflow was decreased to the applicable Rule Curve flow of 6,100 m³/s.

The outflow target for the week of January 3 through January 9, 2026 is 6,170 m³/s, which is 50 m³/s below the Rule Curve flow prescribed by Plan 2014. A slightly lower outflow will be released to offset the minor deviation that was conducted for Lake St. Louis recreational boat haul-out on October 17-18, 2025.  

However, if conditions require, the outflow will be operationally adjusted in accordance with the M Limit.  Since the level of Lake Ontario is above 74.2 m, the first tier of the M Limit applies, and the outflow will be adjusted to maintain a minimum level of 20.64 m at Lake St. Louis (measured at Pointe Claire).

Additionally, during the winter months, the outflow will 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.

On Tuesday, December 23, the Lake Ontario outflow was operationally adjusted from 6,500 m³/s to 6,400 m³/s in accordance with the Plan 2014 M Limit. 

The Rule Curve flow for the week of December 27 through January 2 is 6,100 m³/s. However, the outflow will continue to be set in accordance with the M Limit to start the week.  Since the level of Lake Ontario is above 74.2 m, the first tier of the M Limit applies, and the outflow will be adjusted to maintain a minimum level of 20.64 m at Lake St. Louis (measured at Pointe Claire). 

During the winter months, the outflow will 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. 

As required by the IJC’s Directive, the small amount of additional water (0.34 cm or 0.1 inch) that was released from Lake Ontario during the temporary outflow increase (minor deviation) on October 17 and 18 will be completely restored as conditions permit.