Northern Water Snakes are large, heavy-bodied water snakes that range from 22 to 50 in. There is a great deal of variation in body coloration.
Coloration can be gray, reddish-brown, brown, or black.
The anterior region of the snake has dark crossbands while the posterior end of the snake has dark blotches on the sides and back. The markings run the dorsal length of the snake from neck to anus. The Northern Water Snake's body coloration darkens with age so older snakes may appear to have uniformly dark coloration and the patterning may not be visible. This snake is very similar to the Banded Water Snake but the two species can be distinguished. The Northern Water Snake lacks the dark line that runs between the eye and mouth of the Banded Water Snake .