Josh's advice is right-on.
I had a pond under trees and I can tell you it's a
lot of work if you want fish. I finally gave up after the raccoons cleaned me
out of fish. What a mess to clean, but no maintenance if you let it go, and I
was pretty happy with it. It was still growing plants.
Couple of additions to Josh's list: bird
netting does not keep debris out. Don't underestimate the weight of wet leaves
on the net. Whatever you get has to be strong enough to get the leaves out when
you try to remove them using the netting.
Unless you already have one, hire someone with a
bobcat and or backhoe to dig it for you. It is back breaking work,
especially with existing roots there. Study your tree and see where you can
place the pond with as little intrusion as possible. Don't use the
bobcat/backhoe to tear the roots you do encounter, cut them with a
Shop around for liners. There's some really good
mail-order and I'm sure internet locations. I used a thick liner, roofing
material I think. It was a long time ago. Make sure you protect the liner from
ultraviolet rays if it gets any sun.
Most of all, take your time. Don't feel rushed. I
lived with my imaginary pond laid out with rocks for a year until I was sure of
size, location and shape.
I loved sitting on the bench just enjoying the
water. Like Josh said, if it's the only place you can do a pond, by all means,
do it. If I had to do it over again, no trees. It's an undertaking, and a
commitment, but I loved my pond.
Dan & Lu