Jeremiah 48:8 - Ellicott's Commentary On The Whole Bible

The valley also shall perish... — The cities of “the plain” are enumerated in Jeremiah 48:21-24. They belonged to the Arabah, the sunken valley of the Jordan, the “plains of Moab” of Numbers 22:1; Deuteronomy 3:10; Deuteronomy 4:43. The “valley” here is not connected with anything that helps us to identify it, but it may have been that of the Arnon, or the words may be used generically for “every valley” and “every plain.”