What made the Fall of Foyers a cataract?

Loudon distinguishes rapids as slight, sudden drops in a riverbed; a cataract is a whole river falling over a precipice, while a cascade is water falling from rock to rock in a series of smaller drops^[1].

Her vivid example is the Fall of Foyers: water rushes through a narrow gap between two rocks, then plunges more than forty feet into a chasm, sending up foam “like a great cloud of smoke”^[2].