The brown water snake is very heavy-bodied, and its neck is distinctly narrower than its head. Dorsally it is brown or rusty brown with a row of about 25 black or dark brown square blotches down its back. Smaller similar blotches alternate on the sides. Ventrally it is yellow heavily marked with black or dark brown. Dorsal scales are in 27-33 rows (more than any other North American water snake), and it has 2-4 anterior temporals (usually 1 in others).