Although the term "river of water" probably seemed foreign to Joseph Smith (who was born in Vermont and reared in New York, where rivers are naturally composed of water), the use of the term in the Book of Mormon is consistent with both modern and ancient Hebrew and with other Semitic languages of the Middle East. Different words are used in these languages to differentiate between (1) a riverbed that has water flowing in it and (2) a dry riverbed.
This is one of many examples that prove the Book of Mormon is translation literature. It was not written by Joseph Smith; rather it was translated by him from ancient records.