Short answer. Two years. Article 459 of the Civil Code says that when a river, creek, or torrent segregates a known portion of land and deposits it on another estate, the original owner retains ownership — but only if the land is removed within two years. After that window closes, ownership may shift to the receiving landowner.
What the law says
Whenever the current of a river, creek or torrent segregates from an estate on its bank a known portion of land and transfers it to another estate, the owner of the land to which the segregated portion belonged retains the ownership of it, provided that he removes the same within two years.
Civil Code, Article 459 — Avulsion. Read the full provision →
What avulsion is and how it differs from alluvion
Avulsion happens when a river, creek, or torrent suddenly tears off an identifiable piece of land and deposits it elsewhere. Article 459 covers this: "Whenever the current of a river, creek or torrent segregates from an estate on its bank a known portion of land and transfers it to another estate, the owner of the land to which the segregated portion belonged retains the ownership of it, provided that he removes the same within two years." The distinguishing feature is that the detached piece is known and identifiable — it can be recognized as your land. This is unlike alluvion, where soil deposits gradually and no specific piece can be traced to an upstream owner.
The two-year removal requirement
Your ownership of the detached land survives the flood — but conditionally. The law requires you to physically remove and reclaim that portion within two years of the avulsion event. What "removal" means in practice depends on the circumstances: it may require arranging transport of the soil back to your property, or it may require commencing legal action within the period to assert your rights. The two years run from the time the land was segregated, not from when you discovered it was on your neighbor's property.
What happens after two years
Article 459 does not explicitly say what happens after the two-year window expires, but the logical consequence is that the receiving landowner's rights may prevail — particularly if the detached portion has by then merged with their land or been developed. The receiving landowner would typically hold the land under the alluvion rules or other applicable provisions. This is why timely action matters: delay can forfeit a right that the law would otherwise preserve for you.
Practical steps
If your land has recently been torn away by floodwaters, the first priority is to document the event and the land: photographs, before-and-after survey plans, and records of the boundary lines before the flood. You will likely need a licensed geodetic engineer to survey both your remaining lot and the detached portion on your neighbor's property. If the neighbor disputes your claim, formal legal proceedings may be necessary to assert your ownership within the two-year period. Acting promptly after the flood rather than waiting for the dispute to escalate gives you the best chance of preserving your rights.