I have a mix of sandy loam, loam, and clay loam soils in the location for our new ponds. There is also pure blue clay about 6-8' deep.

I intend to build some forage ponds above grade that will be drained and allowed to dry over the winter.

I originally thought my blue clay would be awesome for sealing these ponds. I now realize the pure clay will almost certainly crack when the ponds dry out and then cause leaks on the next fill cycle.

I am trying to determine the best material to use for these embankment type ponds.

Here is what my trusty resource (Agriculture Handbook Number 590: Ponds - Planning, Design, and Construction) says:

Suitability of a pond site depends on the ability of the soils in the reservoir area to hold water. The soil should contain a layer of material that is impervious and thick enough to prevent excessive seepage. Clays and silty clays are excellent for this purpose; sandy and gravelly clays are usually satisfactory. Generally, soils with at least 20 percent passing the No. 200 sieve, a Plasticity Index of more than 10 percent, and an undisturbed thickness of at least 3 feet do not have excessive seepage when the water depth is less than 10 feet.

In other portions, they state that material with a good mix of grain sizes and 10% clay is sufficient to seal a pond with good compaction. A clay blanket to seal over permeable material should have at least a 20% clay content.

Are there still any "dirt guys" on the forum that know what clay% would be most suitable for embankments/berms that need to be capable of holding water, but not cracking when allowed to dry?

Thanks,
FishinRod