According to news articles in 2012, at 492.5 COE is required to open flood gates!
They will hold the water back waiting for the Trinity down below Dallas to settle down before releasing . In 90 or 91 I seen it at the highest it had ever been. I think it was something like 11' over full pool. At that time they had to raise the flood gates open more than they ever had. I was standing on top of the dam looking down at all the water being released and could fill the dam shaking and rocking and rolling. It did not take long for us to get the hell out of there.
You are right about them having to release the water but only when conditions allow for them to do it without further flooding down stream.