ga pond rookie, I'm talking about a small soil auger:

http://store.jmcsoil.com/catalog/Augers-orderby0-p-1-c-293.html

With the handle extensions, you can get pretty deep and the hole is small. Every 10 foot or so on the downstream toe/backside of the dam should work. The idea is to determine if the water is moving from point A-D or maybe even C-D...I'm sure you get the idea. If the water is moving from A-D and you treat point C with DB-200 then you won't be doing much good.