An farmer ex-neighbour of ours diversified into a trout fishing lake
some years ago. It was excavated with a digger in a level place that
wasn't previously a natural sump or bog, well above a river. He had
expected to use bentonite ( a clay substitute) to line the hole but
found a thick seam of natural clay deep under the topsoil; this turned
out to be good enough on its own.Clay excavated from the base was packed
(by digger)around the sloping soil sides of the hole up to ground
level.I've forgotten the depth measurement; at least 3 metres. The lake
is about three quarters of an acre, filled/topped-up/oxygenated by a
pipe led from a waterfall in the river at a higher level. An overflow
returns water to the river downstream.

During the water-filling, he found there was a leak a few feet down
one side; this turned out to be because the pool wall breached an old
land drain . Once that was stopped off, the pool has successfully stayed
full. The sloping banks were grassed to prevent erosion by people's
feet; that and the high rainfall in west Scotland seems to prevent any
cracks developing where the clay is above water.

Janet.