It's hard to say what's causing the turbidity, but I am leaning toward Tannin staining, as FireIsHot mentioned. Also, as he said, a jar test is your first step toward any solution.
To do a proper jar test, take a clear glass or plastic jar that has can be sealed with a lid, find a spot at least 4' deep, push the jar into the water as deep as you can reach, opening down to prevent water entering, and then flip the jar over to get a less "algae contaminated" sample.
Let the jar sit a few days (checking daily to look for floating matter in the jar) in a dark area to kill any algae present and observe if there is sediment on the bottom of the jar. To compare clarity changes, take photos before and after in the same place/lighting conditions. Preferably with a white background and black striping similar to the pictures shown below.
If the water still appears cloudy, get "Alum" from most any grocery store spice and/or canning supply section. Put in 1/2 teaspoon, seal, shake, and let it sit again. If Alum will clear suspended solids, the water should completely clear within hours.
If after the jar test, the water appears clear, and there were NO floating matter observed when in the dark, the turbidity is being mechanically created through wind/wave action, or critters like turtles, common carp and bullhead catfish.
Starting water sample for clarity comparison....
6-8 hours after Alum added....
About 16 hours after Alum application....