I am looking for data attesting for the variation of fresh water availability in California over the years (ideally 20+ years). For instance river levels, river throughput, wells levels.
If data is not available for California, similar USA data is OK too.
Bonus if the data can be easily used from Python
If you're okay with talking to people, you might want to contact the chief of the Media & Public Information branch of the California Dept. of Water Resources at (916) 653-9712
If you're looking for hydrological data, the USGS has steam gauge data, which has a measure of the height of the water at various locations over time:
For particulate data (which is more a water quality issue), you can try the EPA:
You can infer water availability from total precipitation over a given period. Google has a 115M dataset of weather stats for the US:
