It "looks" like good clay-bearing material in the photos. However, "looks" is absolutely the worst way to evaluate clay content.
Can you go down to the water's edge and grab some of the pond bottom material that is a foot or so above the water line? When it is a little damp, you should be able to roll a nice ball in your hands with good clay. (Try for about the size of a golf ball.)
If you can do that, throw about half away, and try to roll a clay "pencil" or "worm" in your palms. That would indicate an even higher clay content.
You might feel some sand grains in your palms. Do NOT worry about that. Well-mixed sand in your clay-bearing material is just fine.
If you determine you have good clay, then how it was compacted might be the problem. How did they compact your dam? (Running the dozer over it is NOT enough compaction in most situations.) Did they make a core trench?
