Legal Question in Real Estate Law in California

It depends upon what the downhill neighbor means by "responsible." There is a cause of action in California for "unreasonable concentrated discharge of surface waters." The right to sue for this was established by the California Supreme Court in a landmark 1966 case, Keys v. Romley, published at 64 Cal.2nd 396. A downhill neighbor will prevail if his conduct in the matter is reasonable and the uphill neighbor is unreasonably burdening his land with artificially concentrated runoff. But.....keep reading.

In your case, the downhill neighbor probably isn't going to win, because you, or whoever is concentrating the natural water flow into that drain, will probably have acquired an easement by prescription to do what is being done. (I can tell you this with reasonable certainty, because I just lost a case where I represented a landowner in the same position as your downhill neightbor. Of course, key facts may be different! Now, how many LawGuru lawyers ever admit they've ever lost a case?).

A complete analysis would have to include review of the source of the water, the reasonableness of the concentration and discharge by you and others uphill from you, the possible existence of written agreements, and so on. However, keep in mind that under appropriate conditions, long-term continued discharge of concentrated runoff water from Property A onto Property B can result in the Property A and its owner having an easement on Property B for the discharge. Prescriptive easements for discharge of runoff water onto someone else's land are well recognized in California law.