Thanks Cam, do you remember where in
Sepp Holzer's Permaculture he discusses this?
I know he talks in his most recent text about not "fully" sealing a
pond in order to promote the restoration of the hydrological balance. In that, by allowing seeps, the water structures become more balanced across the landscape (this is my feeble reductionist paraphrase).
I wonder if the same is true for the steep slope ponds? It would seem that full sealing is "unnatural", but allowing seeps on a slope seems dangerous!
I was thinking the water would undermine such a structure, built by a less capable person than Sepp Holzer, from underneath and towards the upslope portion of the
pond (with capillary action pulling the water upslope in only a limited capacity). Thereby, creating a slimy, slip layer where the subsoil and/or top soil meets the rock (or other slip layer).
I took an avalanche course a while back, maybe my explanations aren't that great, but I can see parallels (It seems snow might act like water stored in the soil in certain respects?)
I hadn't considered that the area of the pond near the "edge" would give way, since it can be compacted and planted -- something to ponder.