We have a book here done by Mr Aamon Hughes, the man that did work in the Oregon Garden. It has many forms of ponds, demonstrating that the main common reason for ponds is water and the sound of water, rather than a naturalistic look, the latter being "a" reason. When you do this pond, consider adding some water weight on the inside before you fill around the edge, otherwise it can shift or dislodge. If you flush water into sand, it can settle it more. I'd prefer sand for the lower tier where it can be hard to reach with chunky fill.