Where does “川下” come from?

川下 (Japanese) comes from Japanese 川, from Japanese 石川 — Ishi River (rivers in Japan).

川下 (Japanese): downstream; lower part of a river

Definitions

川下 traces back through two separate lines of ancestry.

Step	Language	Word	Meaning
1	Japanese	川	river, stream, brook
2	Japanese	石川	Ishi River (rivers in Japan)

Step	Language	Word	Meaning
1	Japanese	下	the below; the lower portion, end; low in degree...
2	Middle Chinese	下	—