Let me attempt to clarify...
In French, 'seuil' is NOT synonymous with 'slide'. A 'seuil' is a generic term for a sometimes, but not always, essentially river-wide drop of a few feet, where the water does NOT fall vertically. If it does fall vertically, then it is refered to as a 'chute'. I am not sure of the French term for a slide, but 'glissoire' might be it, when for instance only a few inches of water flow over a smoothish rock formation.
For instance, Iron Ring is a seuil, but Quyon is a chute.
Hope rhis helps! --C.