Weekly Regulation Summary
Summary for Week Ending May 21, 2025
| Week Ending: Wed, May 21 2025 | Average this time of the year (c) | |
| Lake Ontario - | ||
| Actual end of week level: | 75.02 m (246.13 ft) | 75.06 m (246.26 ft) |
| Computed Plan 2014 Level (a): | 75.02 m (246.13 ft) | |
| Computed Preproject Level (b): | 75.33 m (247.15 ft) | |
| Weekly Mean Outflow: | 7520 m³/s (265600 cfs) | 7440 m³/s (262700 cfs) |
| Weekly Total Supply: | 8800 m³/s (310800 cfs) | 8040 m³/s (283900 cfs) |
| Lake St. Lawrence at Long Sault Dam Weekly Mean Level: | 73.57 m (241.37 ft) | 73.64 m (241.60 ft) |
| Lake St. Louis at Pointe-Claire Weekly Mean Level: | 21.78 m (71.46 ft) | 21.60 m (70.87 ft) |
| Montreal Harbour at Jetty #1 Weekly Mean Level: | 7.38 m (24.21 ft) | 7.12 m (23.36 ft) |
| Ottawa River at Carillon Weekly Mean Outflow: | 4510 m³/s (159300 cfs) | 2980 m³/s (105200 cfs) |
| Preliminary Lake Ontario Outflow for Week Ending Fri, May 30 2025: | 7610 m³/s (268700 cfs) | 7490 m³/s (264500 cfs) |
Levels are in metres (feet) IGLD 1985. Supply and flows are in cubic metres (feet) per second.
(a) Level that would have occurred with strict adherence to Plan 2014.
(b) Level that would have occurred had the hydropower project, associated dredging and subsequent outflow management not been undertaken.
(c) Historical statistics are based on the available period of record for each dataset: Lake Ontario Water Levels (1918-2022); Lake Ontario Outflows (1900-2022); Net Total Supplies (Inflows) (1900-2022); Lake St. Lawrence Water Levels (1960-2022); Lake St. Louis Water Levels (1960-2022); Montreal Harbour Water Levels (1967-2022); Ottawa River Outflows (1963-2022).
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.