In addition, transplanted animals may be genetically different, and interbreeding with the existing population could produce offspring less adapted to local conditions. At the very least we know that in times like these with very limited resources, they will now compete with the animals already existing at the site, resulting in even worse overcrowding and potentially lowering survival and reproductive success even more than would have happened from the drought conditions.
By law you can take only non-native turtles with a California sport fishing license (California Code of Regulations Title 14, section 5.6), but release into the wild or relocation of an animal (native or non-native) requires a permit from CDFW. In the case of aquatic turtles, the law says it is unlawful to place, plant or cause to be placed or planted, in any of the waters of this state, any live fish, any fresh or salt water animal, or any aquatic plant, whether taken without or within the state, without first submitting it for inspection to, and securing the written permission of, the department (Fish and Game Code, section 6400).
The situation you describe is very unfortunate, but all of California is experiencing a severe to exceptional drought, and many ponds and creeks have already gone dry or are expected to dry by summer’s end throughout the state. This is affecting the state’s wildlife, including many native species that are listed as endangered or threatened under state or federal law.