SkunkedAgain,

I wanted to leave some vertical clay banks in my (proposed) pond to enhance the structure, and some comments said that was possible for good solid clay.

In deep water, I don't see any reason to slope the banks with an excavator. I would dig deep and vertical where possible, since you are going to be fighting for solid ground on excavator placement and you will probably not be able to sit exactly where you want.

Your materials will probably control your slope. Wet sand will just keep flowing into your low spots. A clay-silt-sand mixture could probably stay in place with a 1:1 slope and a little compaction - if you were constructing your pond when it was dry. Solid clay could be sloped vertically. You obviously have a very wide range of possible outcomes.

I think your pond (and the soil and muck in the pond) is only going to allow for steep slopes in some spots. I would get my depth there, and maybe fight just enough in the other spots to determine what is feasible for the areas that will not hold a steep slope.