The common water snake averages 22 to 42 inches in length. It has red-brown to black markings on the back that are bandlike toward the head and become alternating blotches toward the tail. Some adults show no pattern at all. The red or brown markings on the belly extend to the tip of the tail. The body color may be gray to dark brown. Scales are keeled (ridged).
The common water snake may be found statewide in Illinois. It lives in streams, ponds, lakes and marshes. This reptile is aquatic. It is active during the day in spring and fall and becomes nocturnal in summer. It may ﬂatten its body when disturbed and/or release large amounts of nasty-smelling musk from glands at the base of the tail. Mating occurs in May. Males locate females by using their sense of smell. In late summer, the female gives birth to between 20 and 50 young, depending on her size and age. This snake eats ﬁshes and amphibians.