What is the planned max depth of the pond?
The hydrostatic pressure (water depth) is one factor that controls the required thickness of the clay liner.
Also, the dam has to have some lateral strength to resist the pressure of the water in the pond. Imagine a dam built with dry sand and then lined with a foot of clay. It would actually "seal" and hold water, but the pressure of the water would make the sand start bulging in some spot and then the clay seal would crack and then the dam would give way.
Here is an old thread that discusses some of the basics.Pond Keyway
Good luck on your pond! Any project that can kill two birds with one stone is a good project.