Distribution of Giant Gartersnakes (Thamnophis gigas) in the Sacramento–San Joaquin Delta, California, 2018–2019

Open-File Report 2020-1119
Prepared in cooperation with the U.S. Fish and Wildlife Service
By: , and 



  • We examined the occurrence of giant gartersnakes in the Sacramento–San Joaquin Delta, California, in 2018 and 2019. 
  • We made eight captures of seven giant gartersnakes (three females, four males) in 2018, and six captures of six giant gartersnakes (four females, two males) in 2019. 
  • Detection probabilities were exceedingly low despite using methods that achieve much higher detection probabilities in the rice-growing regions of the Sacramento Valley, California. 
  • Our results indicated negative effects of salinity and prey abundance and positive effects of percent emergent vegetation on giant gartersnake occurrence in the Delta, but credible intervals of effect sizes broadly overlapped zero. 
  • Estimates of giant gartersnake probability of occurrence were characterized by substantial uncertainty. 
  • Additional study with a larger sample of randomly selected but accessible sites would help to further resolve the distribution of giant gartersnakes in the Delta and clarify how the physical and biotic environment in the Delta affects where giant gartersnakes exist.
  • Methodological development to increase detection probabilities in the Delta also would improve inference about giant gartersnake occupancy in the region.  

Suggested Citation

Fouts, K.J., Kim, R., Jordan, A.C., Fulton, A.M., Rose, J.P., Ersan, J.S. M., and Halstead, B.J., 2020, Distribution of giant gartersnakes (Thamnophis gigas) in the Sacramento–San Joaquin Delta, California, 2018–2019: U.S. Geological Survey Open-File Report 2020–1119, 26 p., https://doi.org/10.3133/ofr20201119.

ISSN: 2331-1258 (online)

Study Area

Table of Contents

  • Acknowledgments
  • Introduction
  • Methods
  • Results
  • Discussion
  • Summary
  • References Cited
  • Appendix 1 Supplemental Figures
Publication type Report
Publication Subtype USGS Numbered Series
Title Distribution of giant gartersnakes (Thamnophis gigas) in the Sacramento–San Joaquin Delta, California, 2018–2019
Series title Open-File Report
Series number 2020-1119
DOI 10.3133/ofr20201119
Year Published 2020
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Western Ecological Research Center
Description vi, 26 p.
Country United States
State California
Other Geospatial Sacramento–San Joaquin Delta
Online Only (Y/N) Y
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