The red-sided garter snake can grow up to 130 cm. It is black with yellow stripes and red bars on the sides between the stripes. The anal scale is divided.
Mating occurs mostly in early spring when snakes emerge from hibernation but may also occur in the fall just before hibernation. Red-sided garter snakes have live young in late summer or early fall. Groups of females may give birth together at sites which offer protection from predators such as brush or rock piles. In the north, individual females reproduce only every second year, or less often, because of the short summers. Litter size is usually small but the young are relatively large. The young are independent right from birth.
In summer, garter snakes are found in marshy areas where their main food sources, wood and chorus frogs, are abundant. Hibernation sites are often widely separated from summer habitats. This forces the snakes to move long distances each spring and fall. Garter snakes are frequently killed while crossing highways and road mortality may be a serious problem, particularly in the NWT, where populations are small and productivity is low.
The garter snake is among the most widely distributed and abundant of North American reptiles. In the NWT, the red-sided garter snake is known to occur only in the Fort Smith area. The karst topography around Fort Smith provides the crevices and caves necessary for hibernation. Hibernation is often in large groups. These sites appear to provide relatively warm conditions, never falling below 0°C. Water may also be necessary in the dens. These snakes may sometimes submerge during hibernation.