Western aquatic garter snake (Thamnophis couchii)
The western aquatic garter snake is a snake in the family Colubridae with a mild venom for its prey, not dangerous to people. GBIF holds 3,805 licensed U.S. records of it; it reaches our listing threshold in 5 states, most of all California (3,716), New Mexico (24) and Oregon (19). Its observation records peak in June.
Thamnophis couchii, commonly known as Couch's garter snake, the Sierra garter snake, or the western aquatic garter snake, is a highly aquatic species of snake in the family Colubridae. The species is endemic to the western United States, specifically the Sierra Nevada mountain range.
From Wikipedia: Sierra garter snake, text under CC BY-SA 4.0.
Is the western aquatic garter snake poisonous or venomous?
Mildly venomous but not dangerous: the western aquatic garter snake has enlarged rear teeth and a mild venom for its prey, and a bite causes at most local swelling, which is why many sources simply call it harmless. It is not a pit viper or coral snake, the only U.S. land snakes with medically dangerous venom. Clean any bite like a small wound.
Which states have the western aquatic garter snake?
The western aquatic garter snake's U.S. record is concentrated: California alone holds 98% of licensed records. It is listed in 5 states in all; the next are New Mexico (24), Oregon (19) and Utah (16).
| # | State | Records | Share of state snakes | Rank in state |
|---|---|---|---|---|
| 1 | California | 3,716 | 5.0% | 6th of 46 |
| 2 | New Mexico | 24 | 0.4% | 34th of 56 |
| 3 | Oregon | 19 | 0.3% | 12th of 16 |
| 4 | Utah | 16 | 0.5% | 18th of 28 |
| 5 | Nevada | 14 | 0.3% | 24th of 27 |
When is the western aquatic garter snake seen?
Across the U.S., observation records of the western aquatic garter snake run mostly May through September, peaking in June with 26% of them. Almost none are from December to February.
How recent are the western aquatic garter snake records?
The western aquatic garter snake record leans on older material: only 2% of dated U.S. records come from 2017–2026, and the median record is from 1981. The oldest dated record is from 1879; the busiest year was 1979, with 324 records.
| Decade | Records | Share |
|---|---|---|
| 1870s | 1 | 0.0% |
| 1880s | 7 | 0.2% |
| 1890s | 28 | 0.7% |
| 1900s | 5 | 0.1% |
| 1910s | 29 | 0.7% |
| 1920s | 29 | 0.7% |
| 1930s | 134 | 3.3% |
| 1940s | 147 | 3.6% |
| 1950s | 107 | 2.6% |
| 1960s | 96 | 2.3% |
| 1970s | 1,131 | 27.5% |
| 1980s | 1,419 | 34.5% |
| 1990s | 298 | 7.3% |
| 2000s | 545 | 13.3% |
| 2010s | 51 | 1.2% |
| 2020s | 83 | 2.0% |
Species recorded in the same counties
Of the 15 counties where the western aquatic garter snake is most recorded, 13 are also top counties for the California kingsnake, more than for any other snake. Venomous species among its neighbors: the western rattlesnake.
| Species | Shared counties |
|---|---|
| California kingsnake | 13 |
| Western rattlesnake venomous | 12 |
| Gopher snake | 11 |
| California mountain kingsnake | 7 |
| Striped whipsnake | 7 |
| Two-striped garter snake | 6 |
| Western terrestrial garter snake | 6 |
| Aquatic garter snake | 5 |
Related species in the U.S.
Other species of the genus Thamnophis with profiles here: Western terrestrial garter snake, Common garter snake, Western ribbon snake, Aquatic garter snake, Northwestern garter snake, Plains garter snake, Checkered garter snake and Eastern ribbonsnake.
GBIF records read 2026-09-29. Listed: any license. Counted: CC0/CC BY only. How we count.