Weekly Regulation Summary

Summary for Week Ending 22 September 2021

The average Lake Ontario outflow is expected to be 7890 m³/s (278,600 cfs) for the upcoming week, which is the amount specified by the regulation plan. The actual flow during the week may vary depending on conditions in the St. Lawrence River.


  Week Ending: Wed, 22 Sep 2021 Average this time of the year (c) 
Lake Ontario -    
         Actual end of week level: 74.77 m  (245.31 ft) 74.71 m  (245.11 ft)
         Computed Plan 2014 Level (a): 74.73 m  (245.18 ft)  
         Computed Preproject Level (b): 75.43 m  (247.47 ft)  
         Weekly Mean Outflow: 7920 m³/s  (279700 cfs) 7570 m³/s  (267300 cfs)
         Weekly Total Supply: 7250 m³/s  (256000 cfs) 6310 m³/s  (222800 cfs)
Lake St. Lawrence at Long Sault Dam Weekly Mean Level:  72.89 m  (239.14 ft) 73.13 m  (239.93 ft)
Lake St. Louis at Pointe-Claire Weekly Mean Level:  21.15 m  (69.39 ft) 21.16 m  (69.42 ft)
Montreal Harbour at Jetty #1 Weekly Mean Level:  5.87 m  (19.26 ft) 6.19 m  (20.31 ft)
Ottawa River at Carillon Weekly Mean Outflow: 770 m³/s  (27200 cfs) 1090 m³/s  (38500 cfs)
Preliminary Lake Ontario Outflow for Week Ending Fri, 01 Oct 2021: 7890 m³/s  (278600 cfs) 7510 m³/s  (265200 cfs)


Levels are in metres (feet) IGLD 1985.  Supply and flows are in cubic metres (feet) per second.  

(a) Levels that would have occurred with strict adherence to Plan 2014.

(b) Levels that would have occurred had there been no Lake Ontario regulation.        
(c) For comparison purposes, Lake Ontario water level data since 1918 are used to be consistent with those published in the  US and Canadian Great Lakes bulletins (https://www.waterlevels.gc.ca/C&A/bulletin-eng.html). Other averages are for the periods as follows: Lake Ontario outflows and levels at Long Sault and Pointe-Claire since 1960; Montreal since 1967; and Ottawa River outflow at Carillon since 1963.

The regulation plan for Lake Ontario specifies a weekly average outflow from Saturday through the following Friday, inclusive.  To provide timely information for the coming week to the hydropower and Seaway operators, and our readers, we complete the regulation plan calculations each Thursday.  Our calculations use the data available at the time, which are from the previous seven days (Thursday through Wednesday). Since the two time periods do not exactly coincide, their data are usually slightly different.

The table shows the actual flow for the week ending Wednesday.  It also gives the preliminary flow for the coming week ending Friday.  We emphasize that this is the preliminary flow, since unforeseen flow changes may occur after we have issued our notice.  When these flow changes occur, they are reflected in the subsequent week's notice.

Information in this report is compiled from provisional data provided by: Environment & Climate Change Canada, Fisheries and Oceans Canada, Hydro Quebec, Ontario Power Generation Inc, the New York Power Authority, and the U.S. National Ocean and Atmospheric Administration.