Islands which through successive accumulation of alluvial deposits are formed in non-navigable and non-floatable rivers, belong to the owners of the margins or banks nearest to each of them, or to the owners of both margins if the island is in the middle of the river, in which case it shall be divided longitudinally in halves. If a single island thus formed be more distant from one margin than from the other, the owner of the nearer margin shall be the sole owner thereof. (373a)