Here is sort of my rule of thumb with trees on a dam. If the trees on are on top, then they are probably OK, if on the back side of the dam, you may or may not encounter problems, so if the tree is smaller than your arm, cut it down, if larger you may want to leave it where it is. If we can assist you further, you can email me at mikeotto@ottosdirtservice.com or call my office at 940.665.2258 mon to fri from 8 to 12.