Thanks for the replies. I do see the benefit in doing some test holes closer to the dam location to see what's underneath. I think the depth of the rock is pretty consistent around the area, but I'll do some checking to see if one spot is better than another.

Brian, I have the same feeling about the rock bottom. A creek that runs through our property has several pools with solid rock bottoms and they stay full all through the summer (long after the creek has stopped running). I know it's probably less than ideal, but there's not a whole lot that can be done about it short of a liner.

We got quite a bit more rain down there in the last few days, so I'm anxious to get another look at it. I'll try to post some updated pics after next weekend.