FINAL REPORT - International Joint Commission

The definition of extreme events at which point deviations from plan conditions should be considered can
be viewed as somewhat arbitrary. These thresholds can be defined as extreme water level and flow
occurrences approximating 1:100 year exceedance probability events, or by known physical limitations or
response functions of the system, for example Lake Ontario and St. Lawrence River water level conditions
at which significant shoreline damages occur, or levels beyond which environmental benefits may not
continue to accrue depending on recent occurrences of high- or low-water level events. The Study Board
suggests that in the event of a 50% probability that the thresholds listed in Table 15 will be exceeded, the
International St. Lawrence River Board of Control evaluate the effects of deviating from plans flows and
implement appropriate water management strategies, with the express concurrence of the Commission.
FINAL REPORT
Table 15: Threshold Water Levels/Flow
Location Threshold Level/Flow Threshold Level
Lake Ontario
St. Lawrence River
high still-water level 75.6 m low still-water level 73.9 m
(248.0 ft) (242.5 ft)
Outflow at Moses Saunders Dam high flow 9,900 m 3 /s
(350,000 cfs)
Lac St. Louis high still-water level 22.50 m low still-water level 20.2 m
(73.8 ft) (66.3 ft)
Montreal Harbour high still-water level 9.10 m low still-water level 4.70 m
(29.9 ft) (15.4 ft)
The Study Board cannot reach consensus on a position in support of or against this Criterion (k) type of
deviation, nor can the Study Board agree on the magnitude and timing of the extreme event thresholds that
could trigger such deviations. It is suggested that the Commission decide whether deviations for these
purposes should proceed at all, or proceed only with Control Board assessment of effects and/or prior
Commission approval.
A clear procedure to exit from “deviation conditions” should be established in new operational guidelines
that are consistent with the conditions established to enter deviation operations.
Once a deviation action has been taken, successive plan flows should be computed based on actual water
level conditions so that deviation credits or debits are not perpetuated in plan operations.
Status Quo Options
No Action
If the Commission decides not to implement any of the candidate plans developed by this Study (the null
option), then Plan 1958-D remains the written plan under the existing 1956 Orders of Approval, and
deviations would continue under the present authority. Presumably, deviations would be made in a manner
similar to the way in which the Control Board currently deviates, and results/impacts similar to those
modeled by the base case 1958-DD in this Sudy could be expected. However, history has shown that the
Control Board will vary the way it deviates depending on its make-up and the conditions of the time; hence,
some uncertainty surrounds the future impacts of such actions. Under the null option, the needs of the
environment and recreational boaters would not be formally recognized in the Orders of Approval.
Options for Managing Lake Ontario and St. Lawrence River Water Levels and Flows
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