It’s actually a lined snake. Tropidoclonion Lineatum. Lined Snakes are superficially similar to both the Common Gartersnake (Thamnophis sirtalis) and the Plains Gartersnake (Thamnophis radix) in appearance, but can easily be differentiated by the presence of the half-moon markings on the underside.
Got clipped on the tail but he was actually pretty lively and healthy
Sorry for the high crop - I was documenting continuing erosion of the bank and had a surprise cameo apperance.
Snake was found as a neonate. Photographed as an adult in captivity several years later.