These are dark-colored snakes, brownish, tan or grayish in appearance. The back and sides have a series of square blotches alternating with each other that may merge to form bands. Adult snakes can appear solid brown or black, especially when dry. The belly is usually white, yellowish, or orangish with dark half-moon-shaped black edges. Juveniles have reddish brown saddles on a tan, brown, or gray background. Males are usually smaller than females.

Northern water snakes are medium to large snakes, ranging from 61 to 140 cm. They range from 19 to 27.3 cm at birth.

Range length: 61 to 140 cm.

Other Physical Features: ectothermic ; heterothermic ; bilateral symmetry

Sexual Dimorphism: female larger