וְשֵׁ֨ם הַנָּהָ֤ר הַשְּׁלִישִׁי֙ חִדֶּ֔קֶל ה֥וּא הַֽהֹלֵ֖ךְ קִדְמַ֣ת אַשּׁ֑וּר וְהַנָּהָ֥ר הָֽרְבִיעִ֖י ה֥וּא פְרָֽת׃
Y el nombre del tercer río es Hiddekel: éste es el que va delante de Asiria. Y el cuarto río es el 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.