At low tide, the St John River empties into the bay, which is 4m/14ft lower than the river. As the sea level rises and the incoming tidal flow increases, the rush of river water is gradually halted before being gradually reversed, as the water of the bay engulfs the river. This tidal phenomenon is characterised by rapids and whirlpools that are particularly visible where the St John enters a deep gorge before entering the bay.