In researching wandering garters, I found a picture and remembered a story from when I was 18. The picture is here:
Wandering garter snake (Thamnophis elegans vagrans) eating a brown trout, - Image Library

When I was a youth I went on a river trip on the Green river in Utah from the Gates of Ledore to Dinosaur. I being a fisherman brought along my spinning and fly rods and was told by the leader of our expedition who had frequented the river that there were no trout down that low. I didn't care, I was going to fish no matter what. When we left the launch above Ledore, a woman told us that the garter snakes were poisonous and to avoid them. I didn't think anything of it of course. The first night in from Ledore, I carried an 18" Brown Trout draped down my forearm. Everyone was surprised and lots of fishing rods suddenly appeared and people headed up the canyon further.

One of the leaders sons asked if he could gut the fish so I handed him a knife my uncle made (a reputable knife maker), and he set out to cleaning it. Pretty soon he came up the bank with a 10-12" garter draped over the blade from the fishes stomach. I'm not really big on eating fish, but I ate that one that night (maybe it had something to do with the challenge of it).

So pretty soon I started having serious abdominal issues and I was in agony for days. Now I doubt it had anything to do with the snake (likely a wandering garter), but I thought of what that lady had said and regretted eating that fish. I suffered horribly till Jones hole and the outhouse there! It got better from then on.

I always joked that the fish was suicidal. After eating the snake, he wanted to die probably worse than I did after I ate him! So after reading a few scholarly articles on them I wonder if it is at all plausible?

Funny to relate that bad experience (that in all likelihood wasn't really snake related) and being interested in the same species for my daughter years later...

Some toxicity links:
http://vipersgarden.at/PDF_files/PDF-2306.pdf
& "Severe envenomation from "harmless" pet snake:"
Severe envenomation from "harmless" pet snake.
Severe envenomation from "harmless" pet snake.