Thanks for the replies and suggestions therein; I'll keep them in mind as I proceed.
Regarding FishinRod's observation about the clay wall on the right, it has in fact held well for already a year with already water in it at the same height (without a proper liner until now), though we don't get frost here; and in fact I kind of like the look of it as is, as it reminds me of naturally eroded waterside cliffs.
And relatedly to the latter, I feel that esshups's suggestions for substitutes to the clay walls would probably be less pleasing to my eye, but obviously if the clay doesn't hold I would resort to something like that, so I'm thinking maybe I will try with clay first and if it collapses I will go for one of those alternatives, I don't see risk of much damage from such a collapse apart from needing to redo some of the gravel path on the left, as I doubt there would be enough of a collapse to make water pass over the berm on the left or for the whole berm to collapse, though maybe I'm mistaken.
