Tree roots tend to grow towards water. I would think that tree roots from the down stream side of a dam would be those most likely to go through the dam. Trees on the up stream side would have no need to extend their roots through the dam to seek out water.
Conifers have a relative small root ball so should be less of a problem. As a rule of thumb, the roots of a deciduous tree extend only a little further than the tree canopy. The exceptions could be trees like willows, cottonwoods, and silver maples that could have roots searching out water much further out than their canopies.
