A road-killed garter snake found near Haines, Alaska in 2005 has been identified. In a brief note published in the Journal of Herpetology in 2011 (PDF), researches using mtDNA testing identified the snake as a Northwestern Garter Snake, Thamnophis ordinoides. How that snake made its way to Haines, about a thousand kilometres north of its known range, is a matter still to be determined: a previously undiscovered relict population, a natural range extension, and (most likely) an exotic snake either deliberately or accidentally transported and released (e.g., on a ferry) are all possibilities, the researchers say.
The Haines garter snake is literally the only Alaska snake on record: a previous vouchered specimen has since gone missing; other sightings along the Panhandle have not been documented. Via Life Is Short, But Snakes Are Long.