Update for years 2012-2016 (inclusive) is here.
I've been waiting for some time to put this one out. It's an hourly average of lake levels for a five year period.
Each tick on the horizontal axis represents 96 hours. The black line is the normal curve guide while the orange line represents the lower allowed limits, keeping in mind that the absolute lower limit of 645' is always available in emergency situations. I recall seeing the lake that low once, in the 70s, and would prefer not to see it that low ever again.
There are a couple things that really stand out to me, the most noticeable being the drawdown period from February to April. The drawdown is intended to provide time to work on shoreline related projects, such as seawalls, and prepare the lake for the coming spring rains. In the past five years Ameren has not drawn the lake down to the expected low level of 654', instead leaving the lake a full foot higher or more than their own guidance suggests. There is nothing wrong with that of course, but it is a curiously consistent deviation.
The other noticeable time period is the end of August through post-Labor Day. The normal guide clearly shows what most of us expect, a full pool through Labor Day, but the data clearly shows that by the end of August lake levels drop at least a half a foot, recovering a bit before settling down to curve guide level at 658'. Why Ameren does this is something of a mystery. Why not leave the lake at full pool until after Labor Day?
On whole however, the data shows that Ameren does a pretty good job of managing lake levels.