Japanese birch river is Azusa River or simply Azusagawa. Azusagawa is both the Japanese name for the Azusa River, and a Japanese family name.
You learned both of these readings with the kanji. You just need to remember that rendaku turns かわ into がわ.
The Azusa River is usually very calm.
At the weekend, I was soothed by the clarity of the water in Azusa River.
Of course, Azusagawa-san is a talented singer, but the lyrics of her songs are also really good.