Jimmy,
Let me make sure that I understand. The older guides are saying that after the dam was first built, they recall no snails. Then at some later time, snails began to appear lower in the system, and worked their way upstream, and are now at/approaching the dam area? If so, my (weakly) educated guess is below.

I am not a snail expert by a long shot, but I think that there are certain species of snails that are associated with nutrient rich waters, other associated with more sterile waters, cooler waters, warmer waters, and so on. So, without knowing the species of snail involved, there is really no way to know what it means. If I had to guess, I say that it's either an exotic species invasion. or a native that worked it's way down from a cool, spring fed system somewhere, and then worked it's way up the river. Whatever species it is, it can clearly tolerate some pretty cold water.

Maybe try bouncing this hypothesis off the old guides and see what they think. It sounds quite interesting. -- Rick