Short answer. Yes, you keep it. Article 463 of the Civil Code says that when a river's current divides into branches and leaves part of your land isolated, or separates a portion of it from the rest of your estate, you retain ownership of the isolated piece. The split by itself does not transfer title.
What the law says
Whenever the current of a river divides itself into branches, leaving a piece of land or part thereof isolated, the owner of the land retains his ownership. He also retains it if a portion of land is separated from the estate by the current.
Civil Code, Article 463 — Isolated Land by a River Branch. Read the full provision →
Two situations, one rule
Article 463 covers two related events. The first is when the current of a river divides itself into branches, leaving a piece of land or part thereof isolated. The second is when a portion of your land is instead separated from the estate by the current — the water cuts a path around or through it rather than the river forking. In both cases the article gives the same answer: the owner of the land retains his ownership. Being surrounded by water, or cut off from the rest of the parcel by a new channel, does not by itself change who owns the isolated ground.
Why ownership survives the split
The article treats the water's movement as something that happens to the land, not something that takes it away. This is distinct from what happens when a river opens an entirely new bed through private land, where the ground the new channel occupies becomes public. Here, the isolated piece is still your land — it is simply now bounded by water on more sides than before, or physically separated from the main portion of your property by a channel running through what used to be a single, connected parcel.
Practical consequences of being cut off
Article 463 settles ownership, but it does not by itself address how you reach an isolated portion once a channel runs between it and the rest of your land, or the rest of the public road network. Retaining title to ground you can no longer easily walk to is a real problem even though the law says the ground remains yours. Anyone in this situation should have the current boundaries and the isolated portion's exact extent surveyed and documented, since the physical layout after the river moves can be very different from what an old title description assumes.
What this article does not cover
The article addresses ownership when a river's own current divides or separates land; it does not speak to disputes over who caused the change, to the status of the water itself, or to a new bed the river opens through someone else's estate, which is governed by a separate rule with a different outcome. If the situation on your property involves more than one of these events at once — a split, an isolated portion, and also a wholly new channel cutting across neighboring land — each element needs to be looked at against its own governing provision rather than assumed to follow the same result.