Don't know what your project is but am guessing if you need this kind of access across your dam now, then you will also need it in the future. So it sounds like you just need to widen your dam road and ensure they make it strong enough for what you need now and anything you might want in the future. Only other option is to put in a road that crosses below your dam. Not sure which would be less expensive. At least your dam is relatively new and not covered with trees.

"It is the mark of an educated mind to be able to entertain a thought without accepting it"