By way of adding closure to this thread, here's the final results of my "dam repair from behind the dam" experiment:
Out of four leaks, three were successfully sealed by adding clay behind the dam in the leak area (no moisture at all for over a month). Very easy fix, assuming an adequate supply of good clay. The three that were repaired in this manner were part of a new dam segment that had been added to an existing dam.
The fourth leak, which was part of the existing dam segment, was originally caused by critters, e.g. beavers and/or nutria. It has proven to be the most difficult to stop. It may be that the only real fix will be to tear into the old dam and repair the leak at the source.
So, what I learned was that yes, a leak can be repaired from behind with adequate clay, but it is by no means a guranteed fix. Leaks originating from internal dam structure problems probably can not be fixed with this technique. However, other leaks that are associated with new construction, may be repaired with this technique in a manner more economical and efficient than use of other alternatives.
