Dimple Rock rapid ate my lunch LOL as an aside that rapid is formed where the confluence of the river that flows through Frank LLoyd Wrights home :Falling Waters and the Youhioghenny meet.
https://www.youtube.com/watch?v=mrRUbbEGNpE
It is really a combo of things.
the confluence of streams drags yo into "pinball Rock" or Ricochet rock", That sends you careening to dimple rock which has a massive under cut and can suck yo under. Great fun!