20 Moreover, soil which a river has added to your land by alluvion becomes yours by the law of nations. Alluvion is an imperceptible addition; and that which is added so gradually that you cannot perceive the exact increase from one moment of time to another is added by alluvion. 21 If, however, the violence of the stream sweeps away a parcel of your land and carries it down to the land of your neighbour it clearly remains yours; though of course if in the process of time it becomes firmly attached to your neighbour's land, they are deemed from that time to have become part and parcel thereof. 22 When an island rises in the sea, though this rarely happens, it belongs to the first occupant; for, until occupied, it is held to belong to no one. If, however (as often occurs), an island rises in a river, and it lies in the middle of the stream, it belongs in common to the landowners on either bank, in proportion to the extent of their riparian interest; but if it lies nearer to one bank than to the other, it belongs to the landowners on that bank only. If a river divides into two channels, and by uniting again these channels transform a man's land into an island, the ownership of that land is in no way altered: 23 but if a river entirely leaves its old channel, and begins to run in a new one, the old channel belongs to the landowners on either side of it in proportion to the extent of their riparian interest, while the new one acquires the same legal character as the river itself, and becomes public. But if after a while the river returns to its old channel, the new channel again becomes the property of those who possess the land along its banks. 24 It is otherwise if one's land is wholly flooded, for a flood does not permanently alter the nature of the land, and consequently if the water goes back the soil clearly belongs to its previous owner.