The tallrapids are the shallows. I have no idea how this makes sense, but I suppose if the water is taller, it's not going to have as much rapids going on, meaning it's just shallows (but not too tall to become something else, like a lake, or whatever).
Kyoko(Tokyo accent, female)
Kenichi(Tokyo accent, male)
The reading comes from 高い and the reading you learned for 瀬.
The river is pretty shallow around here, so I’m sure you’ll reach the bottom.