FINAL REPORT - International Joint Commission

Plan D + performs well economically in all four stochastic sequences. Its gains are fairly well spread out
among the interests as in the case of the historical and full stochastic. It typically results in the fewest
economic losses of any of the plans. Figure 45 compares Plan D + and 1958-DD under the extremely wet
century S2.
FINAL REPORT
Figure 45: Lake Ontario levels under Plan D + (blue) versus 1958-DD (red) for the 101-year
stochastic sequence with the wettest supplies and biggest range (S2)
Plan D + ’s environmental results are not consistent. While Plan D + shows modest improvements for the
overall environmental index under the historical time series with a 1.10 ratio in the extreme stochastic
sequences, Plan D + only shows a small improvement in the overall environmental index (1.04) under the
S2 wet sequence. Plan D + provides positive meadow marsh benefits in two of the four extreme scenarios
as well as the historical scenario. The 50,000-year stochastic runs for the wetlands model show Plan D +
with only a minor improvement over 1958-DD for meadow marsh despite the 17% improvement indicated
under the historical sequence. Plan D + generally performs better than 1958-DD over the long-term
50,000-year analysis for emergent marsh. However, in the extreme dry scenarios, S1 and S4, Plan D +
performs poorly for emergent-marsh-dependent birds, including some species at risk. To summarize, Plan D +
appears robust and fair in its distribution of benefits for the economic interests under extreme conditions, but
its environmental performance is not resilient, with gains in some scenarios, but losses in others.
Earlier graphs (figures 29-33) show how the candidate plans manage water in some of the driest and
wettest circumstances within the stochastic series. These figures provide comparisons of the three
candidate plans and 1958-DD and present the average levels, the levels exceeded 1% of the time (highs)
and the levels exceeded 99% of the time (lows), in each week of the year, based on the 50,000-year stochastic
sequence. The Study Board used information on these highs and lows in addition to the 50,000-year
stochastic averages and the four stochastic century analyses, to ensure that the plans would be acceptable
in rare, but possible circumstances.
The primary finding of this analysis is that Plan A + has some of the highest average Lake Ontario levels and
some of the lowest lake levels during very wet periods, and it accomplishes this by switching to substantially
higher summer releases than the other plans produce. As a consequence, downstream at Pointe Claire
and Montreal on the lower St. Lawrence River, Plan A + produces the highest and lowest levels. The
outcome is that Plan A + creates shore protection benefits along Lake Ontario but also causes more lower
St. Lawrence River flood damage than any other plan, including Plan E. Plan B + has some of the highest
maximum and lowest minimum levels on Lake Ontario during the extreme conditions, resulting in
somewhat higher coastal damages than the other candidate plans.
Options for Managing Lake Ontario and St. Lawrence River Water Levels and Flows
77