Charles Ellicott Commentary Isaiah 48:18

"Oh that thou hadst hearkened to my commandments! then had thy peace been as a river, and thy righteousness as the waves of the sea:" — Isaiah 48:18 (ASV)

Then your peace would have been as a river. — Literally, “as the river,” that is, the Euphrates, which for the Babylonian exiles was a natural standard of comparison. “Righteousness,” as elsewhere, includes the idea of the blessedness which is its recompense. United with “peace” it implies every element of prosperity.