They could be using a lot of things but glo balls are pretty common there. I've only made it there a few times in the last couple years but have the most success fishing green bead head wooly bugger's without an indicator. Just about everyone I've talked to says the key is to get down deep above the spillway. They keep telling me to using sinking line, but I am always hesitant to break that out (I just don't like fishing with it hence the bead head).
What I usually do is let the fly sink for several seconds, then strip it back a few times, then let it sink again for a second or two, and repeat. So often, when I begin the retrieve again, there is either a fish on or one hits it. I think they pick it up on the in-between falls. Usually can stand next to or upstream from the spillway (not a fan of standing on it) all day doing that and consistently catch fish.