While I am waiting for NRCS to call back, I figured I ask here to see if anyone has had experience with my situation.

I am planning my pond in the bottom of an eroded ravine that is probably 20' deep where the dam will go. The sides are a mix of red clay and sand (according to the experienced dozer guy who will be doing the digging) and that is where my question comes. The dozer guy plans on making quite sure the dam will not leak (his unprompted ideas on coring the dam were a huge relief to me), however my concerns now are on water seeping out horizontally into the eroded hill. He cut some of the dirt from the eroded walls and showed me how the mixture would stay formed in a mass when he squeezed it and said he was not really concerned with seeping into the wall due to the clay that was in the dirt holding the ball together. Does this sound right? I trust the guy, but it never hurts to have others with experience chime in.

Thanks for the help.

Gunny

IMHO a vein of sand, gravel, or other water permeable soil in the gully wall would make a possible leakage path. The presence of clay in the wall does not mean that the leakage path is not also present, although it likely signifies that if the soils in the gully wall were mixed and compacted (a kind of side wall core trench) they would hold water good.

The gully wall is like the ground below a dam in that it presents a possible leakage path for pond water. Unlike on the dam, a fairly massive keyway (to physically prevent the dam from being pushed downstream) is probably not needed (since the sides of the gully are backed by higher ground).

It may be possible to rule out such a possible leakage route from inpecting the sides of the gully - if the visibility of the soils there is good, it would be like a vast test hole.

Gunny, if the dozer guy is indicating that it is sandy loam, you're in business. Like Theo says, your biggest problem would come if there is a pure sand or gravel seam. Aquifers are usually no more than water bearing sand where we pump water from the seepage. As long as you don't hit a starta without about 35+% of clay, it should hold water just fine.

Gunny:
Sounds like your dozer guy knows what he is talking about. Keep in mind that gravity controls the direction of the water and it will not flow uphill.