I agree with western pond turtle. Please release it where you found it. Illegal to possess in California.
"It is illegal to capture, collect, intentionally kill or injure, possess, purchase, propagate, sell, transport, import or export any native reptile or amphibian, or part of one, with very few exceptions in the law. That includes western pond turtles and desert tortoises. (California Code of Regulations Title 14, Section 40)"
Source:
https://cdfgnews.wordpress.com/tag/western-pond-turtle/