http://www.mvk.usace.army.mil/offices/e ... enplot.jpg
If the water ever get higher than the blue line, they run the spillway. If it gets under it, they cut it off. They actually want the red line to stay with the blue one throughout the year...
Like someone said, don't know about the logic to it, but that's how they do it every year.
The other bad thing is they have been following this same blue line since the 70's....