Cottonmouth or Water Snake? Watch How It Swims

If you live outside the southeastern United States, this question has already been answered: there are no cottonmouths where you are, and the snake in the water is harmless. Check your state first — it takes five seconds and resolves the majority of cases.

Within cottonmouth range, four tests separate them, and the first is the best.

1. Watch how it swims

This is the single most reliable difference, and you can judge it from the bank.

Cottonmouths float. They are buoyant, and a swimming cottonmouth rides high with much of its body visible on the surface, like a stick being pushed along.

Water snakes swim submerged. Only the head and a little of the neck break the surface; the body trails underwater and is often hard to see at all.

Two snakes crossing the same pond look completely different doing it.

2. The white mouth

The cottonmouth is named for its defensive display: it opens its mouth wide, exposing a startlingly white lining, and holds the pose.

No harmless North American water snake does this. If you see the white gape, you have a cottonmouth, and you should treat that as settled.

The display is a warning rather than a prelude to attack. A gaping cottonmouth is asking you to leave, which is a reasonable request.

3. What it does when you approach

Water snakes flee. They drop off their basking log and vanish underwater almost before you have registered them.

Cottonmouths hold their ground. They coil, gape, and may vibrate the tail, but they generally do not run. A snake that stayed put while you got closer is more likely to be a cottonmouth; a snake that hit the water instantly is more likely to be a water snake.

4. Head and eye

From a photograph you can also check:

• The facial band. Cottonmouths have a broad dark band running from the eye back across the cheek, often with a pale border above it. Northern and banded water snakes have a narrower dark line; plain-bellied water snakes have none.
• Eye visibility from above. A cottonmouth's brow ridges overhang the eyes, so looking straight down at one you often cannot see its eyes at all. A water snake's eyes are visible from above.
• Head shape. Cottonmouths have blocky, angular jowls and an abrupt narrow neck. Water snakes have a head that flows into the neck — though remember that a frightened water snake flattens its head and can fake this convincingly.

Quick reference

Why this matters beyond your own safety

Water snakes are killed in enormous numbers by people who believe they are cottonmouths. They are harmless, they eat sick and dying fish, and in most of their range there is not a cottonmouth within hundreds of miles.

There is also a practical argument. Attempting to kill a snake is one of the most reliable ways to get bitten — you are choosing to stay in striking range of an animal that would otherwise have left. If it genuinely is a cottonmouth, killing it is the most dangerous available option.

The other candidates

Plain-bellied water snakes are the ones most often mistaken for adult cottonmouths, because both can be uniformly dark. The belly settles it: plain-bellied water snakes have a clean unmarked yellow or orange underside, while a cottonmouth's belly is dark and mottled.

Diamondback water snakes get reported as diamondback rattlesnakes on the strength of the name. Their pattern is an open chain-link mesh rather than solid diamonds, and they have no rattle.