Here is an update. Over the summer I managed to pour one and a half 50 lb bags of bentonite into numerous small cracks in my dam, by making a thick soupy mixture, pouring it into the crack till it was completely saturated and then letting it dry out before pouring more in again.

It's probably a bit early to declare a victory in stopping my loss of water from seepage, but there does seem to be a huge improvement this year compared to last year.

Last spring I had managed to slow down some of the worst leaks, but when the pond was full it would still slowly loose an inch to 1/2 an inch of water a day, for the first 6 - 8 inches, even when there was a lot of water running into it.

This fall my pond has kept every millimeter of rain, with no loss, even if it is dry for over a week. It has even come up 1/4 -1/2 inch after the rain stops because of very small amounts of water seeping in over the clay from the hill behind.

My pond is now 4 inches from full, and things are not yet saturated to the point where water trickles in at the back and the seasonal creek starts to run.

I'm not sure if it is just time that helped settle everything, or my repeatedly pouring bentonite soup into the cracks in the dam that happened over the hot dry summer managed to clog up some of the space that allowed seepage. It may have also been that I repeatedly pounded the 15 inches of clay that got exposed as the pond went down over the summer, using the back of an axe and a sledge hammer, but there is a large improvement from last spring.