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Not exactly sure where you're looking to go, but I can tell you that if it's dry, I would stay away from the Scott's Gulf area (Virgin Falls Pocket Wilderness) and head to the areas where the water almost never stops falling.
For waterfalls that I've never known to dry up, I would recommend Burgess Falls and the falls in the Rock Island area. I wish I could comment on what Big South Fork is like this time of year, but sadly I've never been.
Hope this helps and hope you get some good shots!
Edit: Forgot to mention Fall Creek Falls. When it's flowing, it's really nice, but it does have a tendency to run thin during the dry months. I wouldn't bother if you're unsure about it.
Here's a shot of Burgess Falls in Sparta (not mine)...