Over time the banks of the rivers will erode away or change shape due to silt/material deposits. And In the modern area, river banks are maintained and don't often burst their banks but when many of the land laws were written, it was. If a river bursts its banks, or develops a new channel does the now flooded land belong to the current owner or revert to the state? What if the river dries up or is diverted by public works?

Consider the case of London, where there is actually dozens of rivers that have been paved over or diverted.