This is a jukugo word that uses the on'yomi readings of the kanji. You haven't learned this reading for 谷 yet, so here's a mnemonic to help you:
If you're going to travel through a huge ravine you have to stay hydrated. The best thing to bring is some taste, cold Coke (こく). Nothing quenches your thirst from sweating through a long, rough ravine like delicious Coke!
Have you ever crossed the bridge that spans over this ravine?