FINAL REPORT - International Joint Commission

FINAL REPORT
The plan consists of a water supply indicator, two sets of basic rule curves, seasonal adjustments and a
number of maximum and minimum outflow limitations. The basic regulated Lake Ontario outflow is
derived from a family of rule curves (Figure 1), which show outflow as a function of end of period lake
level and “adjusted supply indicator.” Given the lake level, an outflow is selected for an indicator which is
related to wet or dry prevailing conditions. Seasonal adjustments are applied to the basic regulated
outflow. The resultant seasonal adjusted outflow is compared to maximum and minimum outflow limitations,
which vary throughout the year. If the seasonal adjusted outflow is between the minimum and maximum
limitations for the period, it is adopted as the regulated outflow. If it is higher than the maximum or lower
than the minimum limitation, then the applicable outflow limitation is adopted as the regulated outflow.
1 m 3 /s = 35.315 ft 3 /s
1 m = 3.281 feet
Figure 1: Basic rule curves used in Plan 1958-D weekly outflow regulation
Regulation Criteria
The adoption of Plan 1958-D was based on its successful performance in meeting the requirements of the
ten criteria established in the Orders of Approval (Annex 1). The criteria consider regulated outflows from
Lake Ontario as they affect the minimum level of Montreal Harbour, winter outflows to permit power
generation, outflows during the annual spring break-up in Montreal Harbour and during the annual flood
discharge from the Ottawa River, minimum regulated outflows to secure the maximum dependable flow for
power, and both upper and lower target levels for property owners on the shores of Lake Ontario and the
St. Lawrence River. The criteria also include a statement indicating that attempts should be made to
reduce the extreme high and increase the extreme low Lake Ontario levels.
An eleventh criterion, Criterion (k), was included in recognition that water supplies to Lake Ontario would,
at some times, be outside the range of the historical water supplies upon which the plan was developed.
This criterion specifies that outflows be varied to provide relief to upstream and downstream shore property
4 Options for Managing Lake Ontario and St. Lawrence River Water Levels and Flows