I think it's the tree roots. And when & if the tree dies, it could be another problem.

Basically, I don't think it's recommended to put any trees on the damn itself. If fact, some have said not within 30' of the damn, I believe.

Excerpt from Robert Crais' "The Monkey's Raincoat:"
"She took another microscopic bite of her sandwich, then pushed it away. Maybe she absorbed nutrients from her surroundings."