You beat me to it. The only way to reduce the water level from Henderson Lake when the Atchafalaya is 9 or higher is with a pump or a bucket. Neither one is going to happen.
As Mahtoo said previously 9.0 is the magic number. Any water surface elevation in the Atchafalaya higher than 9.0 fills the Henderson Lake. Sooooo If the control structure is overtoped then having that EXTRA volume available is a good thing to absorb backwater flooding.