DA Aquamaster gave a very conscise briefing about lake diving. To that thorough description, I'll add a few observations since roughly 60 percent of my dives are in small freshwater lakes, reservoirs and quarries.
He's right on about visibility. From the looks of the pix and diagrams in the original post, there are several inlets. These are most likely from streams that flow into the impoundment. These areas often are poor choices for dive sites becuase they are where the streams drop much of their silt load, and visibility will suffer. Generally, I've found that steeper walls along the main channel do better. Also, the closer to the dam the better, since most of the silt will have settled down by this point. However, watch out for the marked areas. Stay out of these.
Thermoclines are usually sharp and distinct. Be ready for wildly varying temps depending upon depth. Go dressed for the occassion, of course.
Be sure to take a knife or other cutting tool. If the site is fished, there will generally be lots of line. If you should become entangled, it's usually a simple matter to slice your way free, but it takes time, so don't press your air limits too closely. Leave yourself some leeway so that you have plenty of time to free yourself without rushing.
Have a partner. I have found that lakes make poor places to solo dive, at least when the conditions such as poor vis and other hazards make things less favorable.
With time, explore deeper parts of the lake. Often, the thermoclines provide distinct zones, some of which will provide better visibility. However, have a good light because even though the zone may be clearer, it will still be dark because of the layers blocking the light from above.
In short, have fun, but stay safe. Lakes can be a lot of fun. Folks who dive "only ocean" may not know what they are missing!