Yes. IDNR lists the timber rattlesnake (state threatened) and eastern massasauga (state endangered, federally threatened) among the four venomous snakes. IDNR
“While venomous snakes are not aggressive and tend to bite people only when stepped on, picked up, or cornered, their bite is a serious matter.” IDNR
Size
Typically 30 to 48 inches (about 3 to 4 feet); recorded up to 75 inches.
How to recognize it
Heat-sensing pit between nostril and eye; no rattle
Head much wider than the neck, with a dark bar from the eye to the jaw; vertical (cat-eye) pupils
Gapes to show the white lining of its mouth when threatened
Where in Illinois
Cottonmouths live in swamps and wet bottomlands in southern Illinois, south of Route 13. IDNR
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251 records by month.What the sources say
Venom: “Four species of Ililnois snakes, the eastern copperhead (Agkistrodon contortrix), the northern cottonmouth (Agkistrodon piscivorus), the timber rattlesnake (Crotalus horridus) and the eastern massasauga (Sistrurus catenatus) are venomous.” (IDNR)
Habitat: Cypress-tupelo swamps, sloughs, and oxbow lakes of extreme southern counties. (Illinois Natural History Survey (INHS), Prairie Research Institute, University of Illinois Urbana-Champaign)
Seen from directly above, a watersnake's eyes are visible but a cottonmouth's are not. Watersnakes have round pupils and no facial pit, and they usually have thin dark vertical lines on the lips.
Alabama DCNR says this snake is often confused with the cottonmouth. TWRA: the diamond-backed watersnake is lighter, with a chain-like pattern; watersnakes in general lack the cottonmouth's facial pits (see the watersnake entry above).
On young cottonmouths the dark crossbands have many dark spots and speckles; copperhead bands have none or one. Adult copperheads keep the banded pattern, while adult cottonmouths are usually plain and dark. A cottonmouth's eye is hidden in a dark facial band; a copperhead's is not.
“Cottonmouths live in swamps and wet bottomlands in southern Illinois, south of Route 13.”
“Four species of Ililnois snakes, the eastern copperhead (Agkistrodon contortrix), the northern cottonmouth (Agkistrodon piscivorus), the timber rattlesnake (Crotalus horridus) and the eastern massasauga (Sistrurus catenatus) are venomous.”
“While venomous snakes are not aggressive and tend to bite people only when stepped on, picked up, or cornered, their bite is a serious matter.”
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