I don't know what to say OSS. It sounds like you did everything the right way. I really thought it was a seam between the old dam and the new dam. It could be the root system of the old stumps. If there is any cracks; a dam that is 26' deep will find it. Another question. When you compacted the soil did you use a sheepfoot roller or just a dozer? One thing I do not like is the damn having a 2:1 slope. I personally would never go tighter then a 1:2 (your dam is 4 times steeper). When any earthen slope is greater then a 2:1 it will slip and i dont care how well its compacted (even with vegetation or concrete rubble).

Trickery and BS, Beats Youth and Inexperience