Mountain Stream
A stream that runs through a valley is a mountain stream. Valleys happen in between mountains, so just picture the river running through the middle of them in your head.
けいりゅう
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
ラフティングで渓流を下りました。
We went rafting down the mountain stream.