וְשֵׁ֨ם הַנָּהָ֤ר הַשְּׁלִישִׁי֙ חִדֶּ֔קֶל ה֥וּא הַֽהֹלֵ֖ךְ קִדְמַ֣ת אַשּׁ֑וּר וְהַנָּהָ֥ר הָֽרְבִיעִ֖י ה֥וּא פְרָֽת׃
At ang pangalan ng ikatlong ilog ay Tigris; ito ang dumadaloy sa silangan ng Asiria. At ang ikaapat na ilog ay ang Eufrates.
Shulchan Arukh, Orach Chayim
Not on every river should you recite the blessing, but rather on the 4 rivers, as it is written "like Chidekel and Euphrates." And he who saw them in a place that wasn't normally walked by people.