Head to head
Cottonmouth vs Northern Water Snake
This is the most consequential snake identification mix-up in North America. The cottonmouth, or water moccasin, is a genuinely venomous semi-aquatic pit viper of the Southeast. The northern water snake is one of several harmless Nerodia water snakes found across most of the eastern half of the continent. Both are thick-bodied, dark, and fond of the water's edge, and enormous numbers of the harmless ones are killed on sight.
| Cottonmouth | Northern Water Snake | |
|---|---|---|
| Venomous? | Yes, a pit viper | No, a harmless colubrid |
| Head | Broad, blocky, distinctly wider than the neck | Narrow, only a little wider than the neck |
| Pupils | Vertical slits | Round |
| Heat-sensing pit | Present, between eye and nostril | Absent |
| When threatened | Often stands its ground; coils and gapes a white mouth | Almost always flees into the water |
| Swimming style | High, head raised, body on the surface | Low, body mostly submerged |
| Range | Southeastern United States only | Most of the eastern and central United States and into Canada |
| Bite | Venomous; painful, tissue-damaging, needs hospital care | Not venomous; bleeds freely from a mild anticoagulant in the saliva |
Cottonmouth vs Water Snake: common questions
How can I be sure whether a snake in the water is a cottonmouth?
From a safe distance, use several features together rather than any one. A cottonmouth has a thick body, a wide wedge-shaped head with a distinct neck, and it commonly swims with its head up and its body floating high; if it feels cornered it may stop, coil, and open its mouth to flash the white interior. A harmless water snake is more slender-headed, swims low with the body underwater, and dives away rather than confronting you. Also check where you are: cottonmouths do not occur north of roughly Virginia and the southern Midwest, so a heavy dark water snake seen in the northern states is a water snake, not a cottonmouth. When in any doubt, simply leave the snake alone.
Why are so many harmless water snakes killed?
Because people assume that any stout, dark snake in or near water in the South is a cottonmouth, and because the water snake's own defensive behaviour reinforces the mistake. A cornered northern or banded water snake flattens its head into a triangular shape, hisses, and strikes repeatedly, which looks alarmingly like a viper. As a result, huge numbers of harmless, beneficial water snakes are killed every year across the Southeast, even though they control fish and frog populations and pose no danger. In much of their range there are no cottonmouths present at all.
Is a cottonmouth bite deadly?
It is a serious medical emergency but rarely fatal with treatment. Cottonmouth venom is mainly tissue-destroying, causing intense pain, rapid swelling, bruising, and sometimes lasting damage around the bite, and it can affect blood clotting. Deaths are very uncommon in modern times because antivenom and supportive care are effective, but bites should always be treated at a hospital and can have long recoveries. Most bites happen when someone tries to catch, kill, or handle the snake; cottonmouths given room will usually retreat or hold still and gape rather than pursue anyone.