The cottonmouth (also known as the "water moccasin") is frequently
confused with any of the Nerodia, most commonly the brown water snake
or the Florida water snake. Often the only way to distinguish these
snakes is the shape of the pupil (round = non-venomous, elliptical =
venomous), presence or absence of the deep facial pit between the
nostril and the eye (no pit = non-venomous, pit = venomous), and
differences in the anal plate and underside of the tail (not
recommended for identification of live specimens). Most water snakes
are non-venomous -- I won't say "harmless," as all Nerodia can bite,
and bite hard, and quite often the bite can become infected due to the
bacteria frequently found in the mouths of snakes. However, virtually
every water snake that people kill is most assuredly a cottonmouth
(just ask them), and is probably *this* (arms outstretched) long. To
add to the confusion, there are many "moccasins" all over the country,
the copperhead is called a "highland moccasin" in many places.
Sometimes people talking about "moccasins" are talking about two
entirely different snakes. I tend to use the term "cottonmouth," then
just about everyone knows to which snake I am referring.

If you live in the South, and you find a snake in your pond and you
are not absolutely sure of its identity, the best bet is to assume
that it is venomous. Don't be fooled by the size, juvenile
cottonmouths often don't have near the venom control of their elders,
and you may well receive more venom from a bite from a juvenile than
you would from an adult of the same species. Then again, you may
receive next to none at all, but it's a pretty silly bet to take.

Sometimes I go to great lengths not to kill critters (I even have
vacuum "wands" to capture wasps, which I release outside) and I
virtually never kill snakes, however the cottonmouth is a special
consideration. There is some argument among herpetologists as to
whether cottonmouths are really aggressive or just very defensive
about what they consider their territory, but it doesn't really
matter, the cottonmouth will approach you and it will bite you. Adult
humans will probably survive with no lasting effects, however it may
be life threatening for a child. Relocation may work if you are
absolutely sure of what you are doing, but then you know all this
already. For most snakes, if you avoid them they will avoid you, and
if you happen to see them, it will probably be as they are crawling
away. In the South, however, it is best to treat any water snake as
venomous. If you can get away, do so, however I know of no way to
co-exist with a cottonmouth if you and the snake both desire the same
spot, and this can be especially critical when children are involved.
--
Galen Hekhuis
I may have mispoken