This water snake is 22 to 55 inches long and the pattern darkens with age, eventually becoming black. The scales are keeled. This snake can be seen swimming among the smartweed during the day and the one shown in the photographs is presumably a juvenile because the pattern is fairly well defined. A similar species, N. fasciata (banded water snake), has a dark line between the mouth and each eye and it is not found in this part of Tennessee.

The northern water snake prefers quiet water and is usually seen close to shore or among vegetation hunting among crevices and vegetation for frogs, salamander, and crayfish during the day or for small fish at night. It has a small hunting range and so may be seen in the same area repeatedly. The snake hibernates in muskrat holes, crayfish burrows or piles of rocks from late October through March. It breeds on emerging from hibernation, and up to 70 young are born from late August until early October. The young are 6 to 12 inches long.

Northern water snakes are often mistaken for water moccasins. However, they are non-venomous although they tend to be aggressive when threatened and will strike and bite, often repeatedly. Bites will bleed profusely due to anti-coagulant in the snake's saliva.