Reptiles and Amphibians of Minnesota by Breckenridge (1944) shows overlap between the two races in southeastern Minnesota. My brother and I encountered this individual on the Cannon River Bicycle Trail in Goodhue County about 4 miles east of Cannon Falls.
Wikipedia also reports that male and female garter snakes produce distinctive pheromones that allow males to find emerging females in the spring. Swarms of up to 25 males mate with each female. Some males, however, practice kleptothermy. Kleptothermy is when you steal someone's heat. In this case, these males release female pheromones, attracting other males to attempt to copulate with them. This action warms up the female-pheromone-releasing males, and makes them more active. Studies demonstrate that these "she-males" are more active than cooler males and are thus able to mate more often. Females are able to store sperm for several years. The fertilized eggs hatch inside the female. Three to 80 young are then born live in a single litter, a condition termed ovoviviparity.