Final answer:
The ownership of the soil depends on the property rights defined for the riverbank areas owned by Smith and Brown. It can be owned by Smith, Brown, or both depending on the property rights or agreements in place.
Step-by-step explanation:
The ownership of the soil that was washed from Smith's riverbank to Brown's is determined by property rights. In this case, it would depend on the property rights defined for the riverbank areas owned by Smith and Brown. If Smith has the property right to the soil that is washed onto Brown's riverbank, then Smith would own that soil. On the other hand, if Brown has the property right to the soil, then Brown would own it. It is also possible that the ownership of the soil is shared between Smith and Brown if they have an agreement that governs the ownership of the washed soil.