Cottonmouth vs Water Snake: How to Tell Them Apart

Key differences at a glance

The cottonmouth (Agkistrodon piscivorus), also called the water moccasin, is a venomous pit viper of the southeastern United States. Water snakes belong to the genus Nerodia, a group of about nine or ten harmless species. The most widespread is the northern water snake (Nerodia sipedon), while the banded, brown, diamondback and plain-bellied water snakes are common in the South.

Both kinds are thick-bodied, dull-colored, semi-aquatic snakes that bask on logs and banks and eat fish and frogs. Adults of both often darken with age until the pattern almost disappears, which is why they are confused so often.

No single feature is foolproof, so it is best to look for several clues together.

Head shape and face markings

A cottonmouth’s head is broad and blocky, clearly wider than its neck, with a flat crown and an overhanging brow ridge above each eye that gives it a frowning look. A broad dark band runs back from the eye toward the angle of the jaw, often bordered by thin pale lines. This stripe is strongest in eastern populations and fainter in the west.

Water snakes have a head that is a bit wider than the neck but usually narrower and more rounded than a cottonmouth’s. They lack the dark eye stripe, and most species have dark vertical bars on the lip scales.

There is an important catch. A frightened water snake flattens its head and body, which makes the head look wide and triangular, just like a viper’s. A triangular head on its own is therefore not proof of a venomous snake.

Pupils and pits

The most reliable differences are on the face, but you need to be close to see them. Cottonmouths, like all pit vipers, have vertical, cat-like pupils and a heat-sensing pit on each side of the head between the eye and the nostril. Water snakes have round pupils and no pit.

In bright light a cottonmouth’s slit pupil is easy to see in a photograph, but in shade it can open wide and look round. These features are best checked with binoculars or by zooming in on a photo, never by approaching the snake.

Pattern and color with age

Young cottonmouths are strongly banded, with dark crossbands on a tan or reddish-brown background, and they have a bright yellow or greenish tail tip used to lure prey. As they grow, the bands fade and many adults become plain olive, brown or black.

Water snakes follow a similar path. Young northern water snakes are crisply banded near the head and blotched farther back, while many old adults look plain brown or black, especially when dry. The yellow tail tip of young cottonmouths is one useful difference, because baby water snakes do not have one. Young cottonmouths are also confused with copperheads, their close relatives.

Swimming posture

How the snake sits in the water is one of the best clues from a distance. A swimming cottonmouth is buoyant and floats high, with much of its body lying on the surface and its head often held up at an angle. Water snakes usually swim with the body lower in the water, so mostly the head and neck show above the surface.

This is a helpful rule of thumb, not a guarantee. A water snake can float on the surface when resting, and a cottonmouth can dive. More detail on cottonmouths in water is in Can cottonmouths bite underwater?.

Behavior when approached

Behavior is often the clearest giveaway. Water snakes are nervous and quick: when a person walks along the bank, a basking water snake usually drops off its branch with a splash and swims away or dives. If cornered or grabbed, it bites repeatedly and smears musk, but it rarely puts on a standing display.

A cottonmouth more often stays where it is. It may coil, vibrate its tail and throw back its head with the mouth wide open, flashing the white lining that gives it its name. Field tests found that cottonmouths usually fled or displayed rather than biting; see Are cottonmouths aggressive?.

Where they live

Range alone settles many cases. Cottonmouths live only in the southeastern and south-central United States, from southeastern Virginia to Florida and west to central Texas and Oklahoma, reaching north along the Mississippi and Ohio valleys to southern Illinois and Indiana. Water snakes live across most of the eastern and central United States and into southern Ontario and Quebec.

In much of the northern United States and all of Canada, any “water moccasin” is a water snake. Within the South, where both live, water snakes are still often the more common snakes along lakes and rivers.

Which is more dangerous?

Only the cottonmouth is dangerous. Its venom causes severe pain, swelling and tissue damage, and a bite always needs emergency care, although deaths are very rare in the United States. Water snakes are not venomous; their bites are painful scratches that bleed freely because of a mild anticoagulant in the saliva.

The sad irony is that harmless water snakes are killed far more often than cottonmouths, because people assume every snake in the water is a moccasin. Killing or handling snakes is also how many bites happen. The safest rule for both kinds is the same: watch from a distance, give the snake room and let it move away. If anyone is bitten by a snake that might be venomous, call 911 (US) and seek emergency medical help immediately.