Read my post on thread "blue clay ".

Dig during a wet time and follow those " water mains " up hill.
If they go into rock before they reach your planed water depth then it might back feed during low water table times like you said. If not then no problem.
Your only problem will be from seeps or springs that come from below water line or under the dam. Dig back into the hill until you hit good dry clay or rock. If the rock is bleeding or gushing water then you will have the problems described. If you can, site the pond down hill from the problem springs. And absolutely never build a DAM on a seep or spring. The WHOLE footprint of the pond MUST be dug down to dry clay or dry rock or you are asking for trouble of many kinds. See the blue clay post.

Your "water mains " are probably crayfish tunnels. they burrow down steam from springs especially in more gently sloping land.
The water flow keeps them oxygenated. I don't think they will dig through your dam as long as it's not leaking but if it leaks they might tunnel toward the source of the water flow and drain your pond so make a good dam that doesn't seep.

Virgin clay with no slip seams will support a very steep angle of repose and will usually support a ledge. I would try 10-20 degrees from vertical. Maybe thirty degrees for well packed clay. I don't have any experience with packed clay ledges but I assume it would be a little less stable than virgin clay.

Last edited by Turtlemike; 03/19/10 04:35 PM. Reason: ledge info