Nevermind, I found the article.

Molecular phylogeography of common garter snakes
(Thamnophis sirtalis) in western North America:
implications for regional historical forces

Skimmed through it, might have missed something, so corrections are appreciated.

The short version of the conclusions (well, one conclusion relevant to this discussion) was that T. sirtalis in the western US are split into three separate groups that are about as closely related to each other, as they are to T. sirtalis further east.