On Wednesday, July 1, 2020 at 5:14:31 AM UTC+12, Algernon Goss-Custard wrote:
We have a low-lying, boggy garden with a stream running through it. Come
the lockdown and Lady Goss-Custard decided to have a pond. So we got a
guy in with a mini JCB and he dug a hole about 5.5m by 3.3m by 1m deep.
There is about nine inches depth of topsoil, below that it is all solid
yellow clay.

Having dug the pond, we ordered the butyl liner, only to find it would
take a couple of weeks to arrive. A few days after the digging, and
before the liner arrived, the rain absolutely chucked it down for a
couple of days, so that the surface run-off filled the pond almost to
the top. The level has stayed steady ever since, two weeks now.

In the meantime the liner has arrived. The dilemma is now: Do we leave
the pond as it is, without a liner, on the assumption that it will stay
full of water and the walls will not collapse? Or should we pump all the
water out, put the liner in and pump water back in to fill it up again -
which we foresee will be a very dirty and unpleasant job now that
everything is sodden? If we do put the liner in and refill, what will
happen in the next downpour - will rainwater get in behind the liner and
balloon it inwards into the pond?

--
Algernon

Clay used to be used before there were butyl liners of course. Leave as is & see how it goes?