Lydia wrote But maybe the
wetlands have some sort of natural drain or process that serves the same
purpose as a bottom drain.

A lined pond is not natural as Mother Nature sees natural. Mother Nature plans
for a pond to fill in over time. Pond to wetland to wet meadow to regular
meadow. If she keeps the water constant in it then it turns into a bog, full of
anaerobic bacteria and it stinks!
I have a bog garden and when one of the labradors goes for a mud bath she
spends the night in the garage! Sure way to turn a yellow lab into a chocolate
lab. ;-)

k30a
and the watergardening labradors
http://www.geocities.com/watergarden...dors/home.html