Legal Question in Real Estate Law in California

You ask a simple question with no simple answer. The broad simple answer is that it depends on how and why the erosion happened and how and why it affected your property. Man-made versus naturally occurring erosion will be a big issue.

For starters, think of this: If both of your families had bought your respective properties in 1830 and no one has set foot on the land ever since. Now you inherit it and then erosion happens.

In that situation, your neighbor has absolutely no responsibility.

Now suppose your neighbor bought their parcel and did a bunch of landscaping and irrigation and between new patios and a pool and landscape drainage, a lot of rain runoff suddenly is concentrated and causes erosion onto your property that never would naturally have occurred.

Absent other contrary details, your neighbor would be 100% liable for any discharge onto or damage to your property.

I would bet your situation is somewhere between those extremes. And that grey zone need an in-person legal consultation.