Slide 3 of 9 Notes:  

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This block of hydro energy is available only if special operating constraints allow the system to draft below AER elevations.  Generally, these special constraints are included to enhance protection for fish by providing higher flows for migration.  This block of hydro energy is relatively small and is generally not available from December to March.  When it is available, it averages only 280 MW.  Over the 50-year record, the largest single month value for this block was about 1,100 MW.