Short answer. Yes. Article 463 is explicit: where a river divides into branches and leaves a piece of land isolated, the owner keeps his ownership, and he keeps it too where the current separates a portion from the rest of the estate. Being cut off by water changes your access, not your 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 →
Why nobody acquires the isolated piece
Accession rewards someone only where land is genuinely added to their property or genuinely lost to identification. Here neither happens. The parcel is still there, still where it always was, and still capable of being matched to the description in a title — the water simply now runs on both sides of it. So there is nothing to allot to the owner across the river and nothing to convert into public property. The rule holds equally for the second situation the article covers, where the current shears a portion off the body of the estate without moving it anywhere.
Distinguish this from land that is actually gone
The article protects land that is isolated, not land that has been carried away or submerged. Where a bank erodes and soil is transported downstream and deposited against someone else's property, what you lost you lost, and what accumulated on the other estate belongs to that owner under the rules on accretion. The line between the two is factual and worth drawing early: is your ground still identifiably there behind a new branch of the river, or has it been dispersed? Only the first situation is answered by this provision.
The real fight is access, not ownership
Owning a piece you can no longer walk to is the practical problem, and confirming your title does not solve it. Reaching an isolated portion generally means crossing the water or crossing a neighbour's land, and the terms on which that can be done are a separate question from who owns the parcel. The channel itself, being a watercourse, is not yours to bridge or fill as you please. Anyone planning to use the isolated portion should treat access as the item to resolve, and resolve it before spending on the land.
Fix the parcel on paper before the ground shifts again
Rivers keep moving, and the argument gets harder the longer the isolated portion sits unvisited. A relocation survey tying the isolated piece to the corners in your technical description is the document that ends the debate about whether it is the same land. Dated imagery from before and after the split does the same work for the cause. Keep tax declarations and payments current on the whole area described in your title, so no gap opens up between what you own and what you have been treating as yours.