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Venomous Water Snakes? Not In Northern Illinois

At the Forest Preserve District, we get asked a lot about if a particular snake is venomous. Our answer is always the same: There are no venomous snakes in Will County. But that doesn't stop many people from thinking every snake they see in the water is a harmful cottonmouth and not a harmless northern water snake.

Northern water snakes like this one are not venomous. (Photo by Chad Merda)

Some of the confusion between these two snakes is understandable. That's because cottonmouths are also called water moccasins, and many people use the term "water moccasin" to refer to all water snakes. Only one of the seven water snakes that live in Illinois is venomous: the cottonmouth. All others are non-venomous and pose no risks to humans.

Here in Will County and northern Illinois you can safely know that any snake you see in the water isn't venomous. That's because cottonmouths only live in the far southern part of Illinois.

Both cottonmouths and northern water snakes, also called common water snakes, have dark bodies, and they typically have bands on their bodies. Both also have keeled scales, which means they are ridged rather than smooth.


Words to know

Venomous: Capable on injecting venom by means of a bite or sting.

Keeled: A characteristic among snakes in which they have ridges on the center of their scales.


Besides geography, there are a few physical clues to help tell the difference between these two kinds of snakes. First, cottonmouths have thicker, heavier bodies than northern water snakes, which are long and thin. Cottonmouths also usually have a neck that is narrower than their heads, while water snakes have necks that are not distinct from their bodies.

Head shape can also be a good clue. While cottonmouths have thick, block-shaped heads, a water snake's head is flat or slender. Cottonmouths will also have an eye stripe on their heads, while northern water snakes do not always have this stripe.

All the venomous snakes in Illinois, including cottonmouths, are pit vipers, a group of snakes that can be identified by pits, or large openings, between their eyes and their nostrils on each side of their heads. Non-venomous snakes in Illinois do not have these openings, which are used to detect body heat from their warm-blooded prey. Pit vipers also have eye pupils that are elliptical in shape rather than round like in other snakes.

Three other venomous snakes live in Illinois: the copperhead, the massasauga and the timber rattlesnake. Like the cottonmouth, none of these snakes live in Will County.

The copperhead lives only in the southern two-thirds of Illinois. The massasauga is state endangered and is only known to live in four Illinois counties — Cook, Clinton, Knox and Piatt. Even within those counties the rattlesnakes are found in only one or two spots. The timber rattlesnake is state threatened and only lives in the southern third of the state and along the Mississippi River.

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