Legal Question in Real Estate Law in California

Yes.

Well, maybe to probably yes. The law applicable to an uphill neighbor artificially altering the natural flow of water onto a downhill property was discussed in detail, and reformulated to a degree, in a California Supreme Court decision in a case called Keys v. Romley, and published with the citation Keys v. Romley (1966) 64 Cal.2d 396. I'm sure that if you put "Keys v. Romley" into a Google or Bing! search, you'll find it. The rules set forth are generally pretty easy for a non-lawyer to read, understand and apply to other situations. In broad general terms, the uphill landowner is liable to the downhill landowner for harm resulting from unreasonable modifications to the natural flow of surface water, but not always, if the uphill neighbor has acted prudently and reasonably to develop and improve his land and the downhill neighbor's response has been unreasonable, the result could be different. In any case, there is no one-size fits all answer to your question; other facts and circumstances would have to be weighed.