Thamnophis Atratus – Aquatic Garter Snake

Thamnophis Atratus - Aquatic Garter Snake information

The aquatic garter snake, Thamnophis atratus, is a harmless snake species found in California and Oregon. The appearance of the snake can vary but it is usually a dark-colored snake with one or three yellow or orange longitudinals stripes along its body. As its common name suggests, the snake is often found near or in waters and feeds on fish and amphibians found in water.


The basic coloration of the aquatic garter snake is variable and ranges from light gray to dark brown and black. The snake has two well or hardly visible alternating rows of black spots on the flanks, depending on the ground color. A broad light yellow or orange stripe runs along the back. A further light yellow or greenish stripe runs along each flank on the second or third row of back scales. The back and/or the side stripes may also be missing. The belly can be blue, green, yellow or white. The head is often lighter-colored than the rest of the body and can be olive-green.

Santa cruz garter snake aquatic snake in california head
An aquatic garter snake with an ant on its eye. Photo: Edward Rooks

They have strongly keeled scales, giving them a rough appearance. Since its range overlaps with the common garter snake (Thamnophis sirtalis), the two species are often confused. Especially the subspecies Thamnophis sitralis fitchi found in California and Oregon can look very similar.


Adult aquatic garter snakes reach an average length of 18-40 inches (45-100 cm). They are considered medium-sized snakes in the United States.

dark black brown snake with yellow stripe on back aquatic garter snake in california
The aquatic garter snake is an excellent swimmer. Photo near Oakland, California from Sarah Stierch

Hunting Behavior and Habitat

The aquatic garter snake inhabits oak forests, grasslands and marshes. It usually lives near water and in open areas that allow basking in the sunlight. The snake can be found from sea level to elevations of 6,000 feet (1,800 m).

When hunting, juvenile snakes lurk near the banks of still waters. They attract fish at the water’s edge by repeatedly touching the water’s surface with their tongues and thus imitating an insect wriggling in the water. Adults are active hunters and prefer the banks of fast flowing waters or deeper waters. They prey on tadpoles of frogs and toads, as well as on small fish and salamanders.

juvenile Santa Cruz aquatic garter snake in oregon
A juvenile Santa Cruz garter snake (Thamnophis atratus atratus) found in California. Photo: Richard Masoner


Like other species of the garter snake, the aquatic garter snake has a mildly toxic saliva that can cause a slight burning or itching sensation for humans or pets. Overall, a bite from the aquatic garter snake is not medically significant or dangerous. When danger appears, the snake will most likely take off quickly into a water stream or a lake or hide under rocks.

Aquatic Garter Snake range in the USA

The aquatic garter snake can be found along the coastlines of the Southwestern United States in California and Oregon.

Subspecies of Thamnophis atratus

  • Thamnophis atratus atratus – Santa Cruz garter snake (found in the outer coastal area of the San Francisco Bay)
  • Thamnophis atratus hydrophilus – Oregon garter snake (found north of San Francisco Bay up to Oregon)
  • Thamnophis atratus zaxanthus – Diablo Range garter snake (found in the south of the range from San Francisco to Santa Barbara)
Thamnophis Atratus - Aquatic Garter Snake information
An adult Santa Cruz garter snake – Thamnophis atratus atratus. Photo: Dawn Ellner

Scientific classification of Thamnophis atratus

  • Kingdom: Animalia
  • Phylum: Chordata
  • Class: Reptilia
  • Order: Squamata
  • Suborder: Serpentes
  • Family: Colubridae
  • Genus: Thamnophis
  • Species: Thamnophis atratus
Thamnophis Atratus – Aquatic Garter Snake

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