Average adult size is 20-48 inches (51-121 cm), record is 74.5 inches (189 cm). A dark-colored, heavy-bodied snake. Juveniles are brightly colored with reddish-brown crossbands on a brown groundcolor. The dark crossbands contain many dark spots and speckles. The pattern darkens with age so adults retain only a hint of the former banding or are a uniform black. The scales are keeled. The broad, dark, facial stripe is not well-defined in large adults. The snout tip lacks vertical dark markings. The head is thick and distinctly broader than the neck, and when viewed from above, the eyes cannot be seen. The top of head in front of the eyes is covered with large plate-like scales. The pupil is vertical (catlike). There is a deep facial pit between the nostril and the eye.
Some people believe cottonmouths lie in wait on tree limbs overhanging water so they can drop into boats. These are usually cases of mistaken identity. The harmless brown watersnake often basks on tree limbs over the water, and when frightened by a rapidly approaching boat, they will escape by throwing themselves off the limb and into the water. Occasionally the watersnake's attempt to flee comes too late and it falls not into the water, but into the boat.
Eastern Cottonmouth: cheek stripe not well defined and the snout tip lacks vertical markings. Range - coastal plains, southeastern Virginia south through the Carolinas, west through central Georgia into Alabama.
Florida Cottonmouth: cheek stripe is distinct, and 2 vertical markings on the tip of the snout. Range - southeastern Alabama, southern Georgia, and Florida.
Found in the eastern United States from Virginia, south through the Florida peninsula and west to Arkansas, southeastern Kansas, eastern and southern Oklahoma, and eastern and central Texas. A few records exist of the species being found along the Rio Grande in Texas, but these are thought to represent disjunct populations, now possibly extirpated. The type locality given is "Carolina," although Schmidt (1953) proposed that this be restricted to the area around Charleston, South Carolina.
This species as being found in Alabama, Arkansas, Florida, Georgia, Illinois, Indiana, eastern Kentucky, Louisiana, Mississippi, Missouri, North Carolina, Oklahoma, South Carolina, Texas, and Virginia.Maps provided by Campbell and Lamar (2004) and Wright and Wright (1957) also indicate its presence in eastern Tennessee and extreme southeastern Nebraska.
In Georgia it is found in the southern half of the state up to a few kilometers north of the fall line with few exceptions. Its range also includes the Ohio River Valley as far north as southern Illinois, and it inhabits many barrier islands off the coasts of the states where it is found
The venom of the cottonmouth is hemotoxic, causing swelling and necrosis near the site of the wound, and potentially death of the victim if treatment is not received promptly. The venom is more toxic than that of its close cousin the copperhead, but nowhere near as toxic as those of rattlesnakes and other vipers. Treatment generally includes intravenous fluids, antibiotics, and an antivenin like CroFab. Like many vipers, the cottonmouth is capable of inflicting what is referred to as a "dry bite", where no venom is injected, but any bite from a venomous snake should be treated as serious and immediate medical attention sought, even if no immediate effects from the venom are felt. Compared to other venomous snake species in its geographic range the venom of a cottonmouth is relatively weak and is unlikely to kill an otherwise healthy human adult. Antivenin is typically only administered in severe cases, and medical treatment can be necessary to prevent complications. The bite however is extremely painful, prone to gangrene, and loss of digits is possible with subpar treatment.