Rapids
A waterfall river is a river that's moving like a waterfall! Fast and furious! Water that moves like this are rapids.
たきがわ
The reading is the kun'yomi vocab reading of each kanji: 滝 and 川. Careful of the rendaku.
祖母に、滝川を泳ぐのは危険だと言われました。
My grandma told me that it is very dangerous to swim in the rapids.