Turtle Resources
There should really be only one freshwater turtle in California. However, many other turtles from the eastern US and elswhere end up in California, typically originating as pets, but end up in the wild. These resources may help ID some turtles that are otherwise rarely seen in California.
Peterson Field Guide
Conant and Collins. 1998. A field guide to reptiles and amphibians, eastern and central North America, 3rd edition (Peterson Field Guide Series). GoogleBooks (plates only, no species accounts): https://books.google.com/books?id=32Rm7c-v_McC&newbks=1&newbks_redir=0&lpg=PP1&dq=fieldguide%20to%20eastern%20reptiles%20and%20amphibians&pg=PA40#v=onepage&q&f=true
Turtles of the World
Hosted by Naturalis Biodiversity Center. I believe this is an internet version and update of Ernst & Barbour 1989 and Ernst et al. 2000.
https://turtles.linnaeus.naturalis.nl/linnaeus_ng/app/views/introduction/topic.php?id=3290&epi=11
This should teach you everything you need to know about turtles. The website is a bit odd to navigate; sometimes you need to click and expand the “Menu” to navigate to other sections. Navigation was the primary issue; you can’t just turn the page and get to the next section. It has detailed species accounts and subspecies. It has a key that is very helpful. By moving through the key, it tells you which species you’ve eliminated and which species remain, both with links to those species; as well as a decision path that you’ve taken up to that point. Beware of older taxonomy and nomenclature. When you are on a species account page, note that there is a sub menu for Description, Classification, Nomenclature, Literature, and Media (photos).
Examples:
Western pond turtle (Clemmys [~ Emys, Actinemys] marmorata) https://turtles.linnaeus.naturalis.nl/linnaeus_ng/app/views/species/taxon.php?id=8095
Common slider (Trachemys scripta) https://turtles.linnaeus.naturalis.nl/linnaeus_ng/app/views/species/taxon.php?id=8103&epi=11
Painted turtle (Chrysemys picta) https://turtles.linnaeus.naturalis.nl/linnaeus_ng/app/views/species/taxon.php?id=8102&epi=11
Catalogue of American Amphibians and Reptiles https://repositories.lib.utexas.edu/handle/2152/44116 (these can also be found on Internet Archive) (taxonomy may be old; the accounts may be dated).
Trachemys, https://repositories.lib.utexas.edu/handle/2152/45343
Trachemys scripta, https://repositories.lib.utexas.edu/handle/2152/45332
Clemmys marmorata (aka WPT), https://repositories.lib.utexas.edu/handle/2152/45027
Chrysemys, https://repositories.lib.utexas.edu/handle/2152/45335 (taxonomy may be old)
Pseudemys, https://repositories.lib.utexas.edu/handle/2152/45310 (taxonomy may be old)
