Al,
Trees so young should be able to be removed from the dam without much worry.
I've have no problems cutting trees of 4" diameter (measured 3 feet from the ground).
At the same time, I would try to figure out how large the root balls are and what impact they might do to the dam. Before cutting, you might seek the opinion of a competent earthmover in the area. Get him on site, let him look and tell you what he thinks.
