tributary

HUCKLEBERRY FINNの本文ではなく、覚えたい単語がまた注にありました。
The State of Illinois lay on the east side of the Mississippi opposite Missouri, in the great angle formed by the river and its tributary, the Ohio.
この "tributary" を辞書で見ます。
・OneLook Quick Definitions: a branch that flows into the main stream
・Dictionary.com: a stream that flows to a larger stream or other body of water.
main streamに対する支流はbranch (stream)だと思っていましたが "tributary" もあるのですね。