It's due to the karst topography. There is very likely a sizable cave that exists beneath the river bed, the entrance to which had heretofore been clogged by s

It's due to the karst topography. There is very likely a sizable cave that exists beneath the river bed, the entrance to which had heretofore been clogged by sediment, which has been washed away by recent rains. Could also be that a choke deep in the cave itself has been washed out, joining it to another cave, which provided enough throughput to drain the river entirely underground.