From 川(かわ) (kawa, “river, stream, brooke”) + 沿(そ)い (soi, “along”, 連(れん)用(よう)形(けい) (ren'yōkei, “stem or continuative form”) of the verb 沿(そ)う (sō).). The soi changes to zoi as an instance of rendaku (連濁).
かわぞい • (kawazoi) ←かはぞひ (kafazofi)?