Like has all ready been stated in this thread. Mild winters is the main contributor. Mild winters mean no ice coverage, which means continual evaporation.
(Now the Global warming debate can begin again.)
They are going to come back. Again, like has been posted earlier, in the 80's we were at a peak in high levels. People were complaining than because than thier docks were under water.
Where has the drastic change come from in the past thirty years? The Controling locks in Chicago haven't changed, the number of gallons per day that we use for drinking has not changed that drastically since the 1980's.
Its just that we are in a low cycle. They will come back.
It took a long time for them to get this low, its going to take a long time for them to recover.
Courtney is My Hero
If a man is to be obsessed by something, I suppose a boat is as good as anything, perhaps a bit better than most - E.B. White