FINAL REPORT - International Joint Commission

To reduce the risk of Lake Ontario and St. Lawrence River flooding in the following spring and summer, it
makes storage available for reduced flows during the Ottawa River freshet. It does this by applying a rule
to adjust releases whenever the Lake Ontario level is above 75.0 m (246 ft) at the beginning of September,
and it strives to lower Lake Ontario to 74.8 m (245.4 ft) by January 1. If the one-week forecast of the
Lac St. Louis level at Pointe Claire indicates the gauge may rise above flood levels, the plan applies a 3-tier
rule that attempts to balance upstream and downstream flood damages. If Lake Ontario is less than
75.0 m (246 ft), lake releases are constrained to keep levels at Pt. Claire below the alert level of 22.1 m
(72.5 ft). If Lake Ontario is at or above 75 m, but less than 75.2 m (246.7 ft), flows are limited to keep
Pt. Claire below the action level of 22.33 m (73.3 ft). Above 75.2 m (246.7 ft), lake releases are limited to
keep Pt. Claire below 22.5 m (73.8 ft). This rule uses a one quarter-month forecast of Ottawa River and
local tributary inflows.
FINAL REPORT
The Plan then checks to see if its releases need to be reduced to limit flows through the Hydro Quebec
Coteau control structure for safe operation. Given a perfect forecast of Lake St. Francis local inflows and
of the maximum capacity of Beauharnois, Lake Ontario releases are reduced to limit flows through the
Coteau structure to 2,500 m 3 /s (88,300 cfs) during ice conditions, and 4,000 m3/s (141,300 cfs) otherwise.
Finally, the Plan checks to see that water levels at the Iroquois Lock will not exceed 75.6 m (248 ft).
This final constraint overrides all others to preserve the ability to control lake releases.
Plan D + : Blended Benefits
Plan D + is a benefit balancing plan combined with short-term forecasting of contributing water supplies.
The intent of this Plan is to increase the net economic and environmental benefits of regulation compared
with those of Plan 1958-DD without disproportionate loss to any interest.
The benefit balancing considers the needs of the interests in the Lake Ontario–St. Lawrence River system
downstream to beyond Montreal. A set of seasonally varying mathematical relationships was developed
based on the expressed water level or flow preferences of the interests and other information used in the
performance indicators in this Study. These relationships assign a score to a particular flow or water level
at one of a number of sites on the Lake or River. The closer the water level is to the preferred level at a
location for the given time of year, the higher the score. The rate at which the score in each relationship
changes as the level rises above or falls below the optimum was adjusted using the performance indicators
of this Study. In order to maintain stability of Lake releases, another relationship reduces the score as the
week-to-week flow fluctuation increases. To improve key wetland habitat diversity, which requires periodic
years of lower Lake Ontario levels, the preferred Lake Ontario level in the growing season is lowered if there
has not already been a period of low levels in the past twenty years and water supply conditions are favourable.
These relationships are used together in a quasi-optimization approach to determine Lake Ontario releases
in each quarter-month or week. Releases are constrained by ice formation and ice roughness factors and by
multi-stage minimum and maximum flow limits that vary with the hydrologic supply conditions. For details
on the relationships and limits used in this Plan refer to the plan description in Annex 3 to this report.
This Plan chooses its releases based on the maximum total benefits indicator score. The plan begins by
checking on the forecasted water supplies to Lake Ontario, the Ottawa River and local flows to Lac St. Louis,
the annual net total supplies, and the ice roughness and cover for the coming week; it then calculates the
smallest trial Lake Ontario release (typically, the present flow minus 400 m 3 /s (14,100 cfs) or the minimum
flow limit, but this may be less if needed for ice formation or in more extreme level conditions). With the
forecast conditions and the trial release, the plan then calculates the trial water levels for Lake Ontario and
the St. Lawrence River using known stage-storage and stage-discharge relationships. It then calculates
the benefit indicator for each relationship for this trial flow and applies the optimization factors and sums
the individual benefit indicators to determine the total score for the trial flow. If the trial flow is less than
the maximum flow (typically, the present flow plus 400 m 3 /s (14,100 cfs) or the maximum flow limit, but
this may be more in extreme level conditions), the plan increments the trial flow by 10 m 3 /s (350 cfs) to
obtain the next trial flow and repeats the process until it finds the release with the best total benefit
indicator score.
Options for Managing Lake Ontario and St. Lawrence River Water Levels and Flows
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