Thanks Bill and Rad. I think this thread would greatly benefit from any comments made regarding stabilization of walls to prevent collapsing inward. Especially in a freeze/thaw climate. I'd like the pond to look as natural as possible, so of course I'd like the upper margin to be level with the surrounding terrain. My intuition tells me that the walls could be made stronger by intelligent use of surrounding soils. After all, they do this to build houses right?
