It looks to me like maybe the water was going over the top of the dam and it started washing it away from the toe area, and was slowly chewing it's way to the pond side of the dam.

What do you have for a primiary overflow, secondary overflow, and how much freeboard do you have in the pond?

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3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).