What does the dozer operator say?
Have you talked with other pond builders?
How about your local Conservation experts (federal and state?).
What was used for the dam? Was it good clay? Did he layer and pack it as he built? Or did he just use silt (complete with weeds, brush, trees) that had washed in and grown over the years.
Did he dig a 'core' so the new dam was 'keyed' into the old dam?
I do not think the weeds or brush would have caused it to fail unless they were so thick he could not pack it. They would have caused problems later from settling and seepage as they rotted.
Did the overflow overtop the dam?
Assuming he used good clay, I'd guess he did not do a good job of packing it as he built the dam.
The lack of a core or keying would give you a problem with seepage, I do not think it would cause the dam to fail right away.
Let us know what you find out and what solution you come up with.
Good Luck.
