Thanks Pat,I've read here about soil saturation and wicking, so I was paying close attention to that, we watched the levels for about 2 months and also pumped water from the seasonal creek until it quit flowing. Hopefully we had enough saturation of the soil to get an accurate read.
I think one mistake I made was not digging the core deep enough. In the Nov./Dec.2018 issue of Pond Boss Michael Gray was talking about my exact situation, hitting clay and not going all the way to bedrock. We went all the way to solid rock on the North end of the dam, the creek channel was in the middle and the south end was clay. He was saying there can be a thin dirt layer between the clay and rock that can allow water through. I had noticed this dirt layer on top of bedrock at the place we were borrowing clay from 18' deep and that was where the water was traveling.The clay from the surface down to the dirt was dry. Amazing how water will find a way to escape.