Mike,
Is this area near the skimmer and do you think all the slumping is from ground water?
EPDM does have quite a bit of stretch.

lsst - I am stymied. Consider the pond as an oval. The skimmer is on one end. The shelf starts near the skimmer (after that slumping), runs all the way along one long side and around the end opposite the skimmer.

When I started the pond back when, I knew I had a very, very wet patch of ground (water within 1" of the surface), but it was what I had. When I built, at the cost of more depth, I installed three 4" perforated drain pipes under the pond, connecting to our storm sewer system.

I just think there is more water than can be handled, vast amounts of ground water and maybe some artesian effect as well. We have an old artesian well in the lower level of our garage, uphill, so there is a clue.

I'm thinking that whatever the source, there is constant ground water keeping the very messy clay wet and it is now slumping. Or it could be bad ju ju from the plants and fish I swore never to have lol

I guess another drain along the pond is next. Looking at a trench 20' long up to 30" deep. Property not accessible by any machinery. I could throw a raucous party for hardy ponders, kayaking and body surfing in the streams.

The epdm is stretchy but now I am concerned that the stone pond sides may collapse as the shelf is undermined.

In retrospect, I should have put in retaining walls, but I thought the drainage scheme was sufficient.

Mostly I can sit back and just hope that I won't hear the splashing collapse in my life time.

Ideas welcome.
Thanks Mike