I am still in the planning phases for a few pond sites.. but trying to read and understand before I start moving dirt...
On my property, I have recently dug for soil samples and to get dirt for covering culverts. I noticed in several places I have dug, I hit good clay but with seams of ground water running thru. Some spots where I have dug have had nearly 1.5 inches diameter holes that would rush in water at a rapid rate and quickly fill my small hole. Almost like I had hit a water main... this has occurred anywhere from 1 foot to 3 feet below the surface and some areas where I wanted to dig my pond.
the Idea of ground water is great for helping keep my pond full, but how do I plan for water to come into my pond without having water leave via these same holes when the water table drops? do I just compact clay into the seeping areas?
also I was planning on having some area with seep drop offs in my pond for ledges... How do I compact near vertical walls for my semi excavated pond?
thanks
Bryan

This is one of the most difficult questions to answer.
Not sure there is a good answer. Look at the other ponds around your area and see what they do when it gets dry-- how much do they go down, when it gets dry?

Do not know any way to pack dirt on a vertical bank.

Have you planned your depth yet?
Talk to local dirt guy and see what he says about the water table.

The truth is if the ground is that wet you may not have any trouble with much water loss.

otto,
To some extent... I am in louisiana and everything holds water.. ha ha.. But really I am in "hills" north of baton rouge.
The few ponds near me are old and poorly build with little water shed and all loose 2-3 feet of water during the dry periods.
One test hole I dug was 4 feet deep by 3 feet in diameter. It completely all but 8 inches with ground water, after a rain completely filled then dropped back to that 8 inch level. However it is directly "up stream" of an old pond that I cut the dam to drain that had nearly completely silted in. There had been ( and continues to be) a continous influx of water seeping from the ground thru the old pond bed and out thru the dam where I cut it.
My dam at this site will be 10 feet total height from current ground level with some excavation to about 15 feet in places. and I am going to have to plan to dig at least 3-4 feet in most areas to achieve depths of 3-6 feet thru the entire pond bottom.
I know no dirt guy around..

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.