I've seen large boulders the water pushed out of the way. So, what may happen to the rock? I would answer, it depends on the shape and size of the rock. Also, are we talking about still water, slow river or fast stream? What is the shape and inclination of the river bed? There's a big chance that the powerful stream will push the rock out of the way. For how long? For as long as the force of the resistance is less than the strength of the water.