Derek Broughton wrote:

If you've got water on both
sides, though, there's no pressure differential.

You would think so, but if you take a shovel full and throw it in the
pond, it will sink to the bottom, so it is heavier, but heavier. Sandy
or muddy soil will always slump since it is heavier than water.
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18,000 gallon (17'x 47'x 2-4') lily pond garden in Zone 6
Cheers, Steve Henning in Reading, PA USA