welcome to PB auburnbob, good to see another northern californian here.
a keyway will not reduce hydrostatic pressure. that is solely dictated by the volume of water in pond and the force it exerts on the substrate of pond.
nice link JBL.
auburnbob, think of a keyway as a core trench dug into the native surface (below the dam or berm that is about to be built)in which dam materials are compacted in lifts, and continue to be brought up above the core trench to form the dam or berm creating one continuous barrier that is effectively jointed into the ground.
it will help reduce seepage below (underneath) the dam. if yer seepage is from within the berm itself, you probably need to groom and repack the berms.
