This picture makes it relatively easy to count the upper labial scales, which is one of the differences between different species of garter snakes. T. elegans has 8 upper labial scales, whereas, the eastern garter snake usually has 7 upper labial scales. One can also see the keels on the snake's scales.

Of course, Internet users have provided some good references: Wandering Garter Snake (aka Western Terrestrial Garter Snake) (common names are always a problem), Eastern Garter Snake (aka Common Garter Snake).