So I went out and measured again, one week later and lost about 3". It's been really hot, so evaporation is kicking up (more on that in a minute). Behind the dam still has moving water. So in my case the Soilfloc has had no effect whatsoever.

As to whether there happens to be a spring that has it's exit right where my dam is, I suppose anything is possible, but that seems unlikely.

As for evaporation, I've checked. I'm in Texas, quadrangle 709:
http://midgewater.twdb.texas.gov/evaporation/quadrangle/709/evaporation-tabular.txt

Last December I my evaporation rate was 1.73" for the entire month. Worst month of the year is July at about 7". That's less than 1/4" a day in July and should be about 1/10th inch in the fall/winter. If you look at my data you can see that my leak is about 1/4" per day and then evaporation is on top of that. And it tracks pretty close to the data in that table.

I'm not sure what my next step will be, but it won't be Soilfloc. I'm pretty disappointed that I applied at a higher rate than recommended and got at least 20' past the bottom of the dam and saw no improvement. I was able to apply it heavier and over more area because the pond shrank quite a bit before I got to apply it.

I've also not seen any evidence of the Soilfloc at the bottom of my pond. Some folks mentioned a hard crust they saw develop, but I don't see that.

I'm not saying that Soilfloc can't work. Maybe it can and I just didn't get it at the right spot. Or maybe it doesn't really work all that well. In my case it was certainly the latter.