Shallows
noun
The tall rapids 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).
たかせ
The reading comes from 高い and the reading you learned for 瀬.
この川はこの辺りは高瀬になってるから、きっと足が底に着くと思うよ。
The river is pretty shallow around here, so I’m sure you’ll reach the bottom.