I have seen this done on a client's ranch. They built dam and then made a concrete wall on backside of dam to protect tree. They used concrete bags and overlapped them to create a wall. This project was concluded about 4 years ago and so far trees are in perfect condition. In fact they get a little more water than rest of trees. Trees are no more than 20 feet from core trench. As Rockytopper says you want to stay away from piling any soil underneath canopy of tree. This is why a wall that separates dam from tree works well.

Fisheries Biologist, Texoma Hatchery.
www.texomahatchery.com