is a small to medium sized gartersnake with a total length of 46-112 cm (18- 44 in). Olive to brown in color with no (or only faintly present) back or side stripes, but with distinct dark brown, dull brick red, or blackish spots on back, that fade on tail. No well- developed stripes or pale crescent behind corner of mouth as in some species of gartersnakes. Brownsih gray below, paling on throat. Venter usually brownish-gray, often with two rows of blackish wedges that fade posteriorly. Because of this drab coloring and high set eyes, this species more closely resembles Nerodia
(watersnakes) than other Thamnophis
. Also, the head is narrow and more elongated than most other Thamnophis
. The tongue is black. Scales keeled, usually 21 rows at mid-body. Anal plate usually single but can be divided. Eight upper labials; dark bars on labial scales. Young have a dull yellowish belly, and often a cream-colored throat.
Found in, or next to, well-lit sections of clear, cool, permanently flowing, rocky streams in areas of pinyon-juniper, oak-pine, or ponderosa pine, commonly sheltered by broadleaf deciduous trees (cottonwood-willow). Seeks cover under rocks in water when disturbed (Stebbins 1985). Inactive in cold temperatures or extreme heat. Almost strictly aquatic, seldom seen more than a meter from water.