the only way to become really familiar with the graphs for, say the 4-5 rivers you fish is to watch them constantly while spending time on the water fishing, when you are successful remember that magic number and dont forget it. When looking at the weekly data compare the levels to the previous weeks/months/years to get an idea what level its at compared to high and low water periods. In As said above, when there is a steep upward spike it usually means muddy water, but always compare to older data because what looks like a big spike can turn out to be a 3cm rise. There are some larger rivers that rise and dirty up much more gradually, some are always fishable and dont dirty up. Every river is unique in some way and experience on the water is important to learning how they act, with some a "blowout" is 4 meters higher then normal, while some it is only 1-2 feet and some smaller creeks only 10-20 cm. When you become familiar with your river(s) you can get a pretty good idea of what point they are fishable when on the drop (that being said, there are still those times when you are completely wrong).
For the "South" area, 7.5-7.8 is about as low as it gets, in early fall when the water table is low a big spike might be to 8.2-8.3 with the water being unfishable, while in the winter months when it is holding lots of water it is usually on the clear side at that level. It can have fishable vis sometimes even when its over 9. If you look on the graph at the April blowout there, thats probably about the highest you will see it, while the data from summer 2012 shows you (likely) the lowest levels ever seen