FINAL REPORT - International Joint Commission

ANNEX 3
The further away from the target level the poorer the score, with the score for levels below the target
decreasing faster than for levels above the target. For example, if the expected level is 0.20 m below the
target level, then the Lake Ontario level score is about -12.0. If the expected level is 0.20 m (0.66 ft) above
the target level, then the Lake Ontario level score is about 27.0. As can be seen from Figure B-19B, the
score decreases exponentially as the expected level deviates further from the target level.
In the development of Plan D + , both the normal target level and the deviation scoring curves were adjusted
many times in consideration of several different interests both on the Lake and the River, including the
ecosystem, until this final set of curves was achieved.
In order to periodically provide lower Lake Ontario water levels in the growing season, Plan D + checks how
many years it has been since the peak annual Lake Ontario level was below 74.70 m (245.08 ft), which is
the peak growing season level that the Environmental Technical Work Group has suggested is needed
periodically to maintain adequate wetland habitat diversity. If it has been more than 20 years since the last
peak annual Lake Ontario level lower than 74.70 m (245.08 ft), then the plan at the end of February checks
whether the Lake Erie outflow (averaged over the past 2 months to smooth out weekly fluctuations due to
wind and ice) is in the “right zone” for there to be a good chance that supplies are apt to produce a peak
Lake Ontario level less than 74.70 m (245.08 ft) in the coming spring-summer period. If the Lake Erie
flows are too high (outside the right zone), then it is unlikely that the Lake Ontario level can be maintained
below 74.70 m (245.08 ft) in the growing season, and it makes no sense to try to reduce the level due to
the impact on the other objectives. If the Lake Erie flow is too low, then it is likely that the peak Lake
Ontario level will be less than 74.70 m (245.08 ft) anyway, and it would be counter-productive to try to
drive the level even lower. If it has been more than 20 years since the last annual Lake Ontario peak below
74.70 m (245.08 ft) and the Lake Erie outflow is in the right zone at the end of February, then the plan
shifts the Lake Ontario target levels down by 0.35 m (1.15 ft) from March through the end of July, after
which the target level is gradually returned to its usual value over the next four months. A gradual, rather
than a rapid return to the normal target Lake Ontario level is made in order to avoid causing a large
reduction in the release that would result in a sudden drop of levels downstream and a rise in Lake St.
Lawrence. The shift is not applied throughout the year since this would exacerbate low levels on the Lake
and the lower river late in the year, with no benefit to the environment. At the end of the year, the plan
checks whether the peak Lake Ontario level in the growing season was indeed below 74.70 m (245.08 ft); if
so, it again checks whether the Lake Erie flows are in the right zone at the end of the following February. If
both of these conditions are met, then the second year shift is applied, with Lake Ontario target levels
shifted down, this time by 0.20 m (0.66 ft), but again for the same March-July period, and then gradually
returned to their usual values over the next four months. This second year of lower peak growing season
level is again provided to better meet the stated wetland habitat diversity needs. Whether or not the shift
was applied, once the peak annual Lake Ontario level has been lower than the 74.70-m (245.08-ft) level for
two years in a row, the year counter is reset and the low shift will not occur again until both the 20-year
and the right Erie outflow conditions occur.
To provide better levels for Lake Ontario recreational boating, the scores shown in Figure B-20 are applied
from the second quarter-month of April to mid October. Outside this period, a score equal to 6/35 of the
Lake-Ontario-deviation-from-target-level score produced using Figures B-19A and B-19B is added.
190 Options for Managing Lake Ontario and St. Lawrence River Water Levels and Flows