Noun and の adjective
ほんりゅう
このフグは、トーフ川本流のワニカニはしの下でつれました。
I caught this blowfish underneath the WaniKani bridge in the main stream of the Tofu River.
コウイチは、ファッションの本流に大きなえいきょうをあたえることで有名です。
Koichi is known for having a big influence on mainstream fashion.
日本文学の本流と言って、最初に思い浮かぶのは誰ですか。
When you say mainstream Japanese literature, who is the first author that comes to mind?
The origin stream, that's where everything comes from. That is the mainstream.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.