The venomous Water Moccasin, also known as the Cottonmouth (Agkistrodon piscivorus), does not inhabit Colorado. This pit viper is exclusively native to the warm, humid environments of the southeastern United States. Any snake encountered near water in Colorado is a different species. The absence of the Cottonmouth is due to specific geographic and climatic factors that make the state an unsuitable habitat for its survival.
The Definitive Answer: Range and Distribution
The Water Moccasin is a semi-aquatic viper whose natural distribution is geographically restricted to the Coastal Plain and lower Piedmont regions of the southeastern U.S. Its range extends from southeastern Virginia, south through Florida, and west to eastern Texas and parts of the Mississippi Valley. This snake requires a specific thermal environment, thriving in areas characterized by slow-moving, warm, shallow water, such as swamps, marshes, and sloughs. The species is highly adapted to these lowland, subtropical conditions, which are entirely absent in Colorado.
Colorado’s environment, characterized by high elevations, cooler temperatures, and significant periods of freezing, is incompatible with the Cottonmouth’s biological needs. The high-altitude, cold-water streams and mountainous terrain do not offer the necessary habitat or the prolonged warm season required for this reptile to successfully breed and hibernate. The state falls far outside the established biological range of Agkistrodon piscivorus.
Snakes Commonly Confused with Cottonmouths in Colorado
The aquatic or semi-aquatic snakes actually found in Colorado that are often mistaken for the Cottonmouth belong to the genus Thamnophis, commonly known as Garter Snakes. Several subspecies are widespread across the state, including the Terrestrial Garter Snake (Thamnophis elegans) and the Plains Garter Snake (Thamnophis radix). The Red-sided Garter Snake (Thamnophis sirtalis parietalis) is often restricted to wetland and riparian habitats along the South Platte River and its tributaries.
Garter Snakes are harmless to humans. They are often found near water because their diet consists of amphibians, fish, and earthworms, leading to frequent encounters with people. When threatened, Garter Snakes may exhibit defensive behavior, such as flattening their head and body to appear larger. This action can temporarily give their head a misleading, triangular shape similar to a pit viper’s.
A key difference is that Garter Snakes have a slender body, unlike the Cottonmouth’s thick, heavy body. While Colorado does have venomous snakes, primarily Rattlesnakes (Crotalus spp.), these species are terrestrial and are generally not found in aquatic environments. Recognizing the local Garter Snakes, which typically feature pale stripes running the length of their bodies, can help alleviate mistaken fears when enjoying Colorado’s natural areas.