There are a lot of fish swimming in this river.
Japan's longest river is Shinano River.
Whenever I see a river, I get so pissed off.
When a vocab is made up of a single kanji with no okurigana (hiragana attached to the kanji) it will usually take the same meaning as the kanji it comes from. So, from that you know that this word means river.
You're in luck - the reading you learned with the kanji is the same as the one you have to learn for this vocab!