Charles Ellicott Commentary Genesis 36:37

"And Samlah died, and Shaul of Rehoboth by the River reigned in his stead." — Genesis 36:37 (ASV)

Rehoboth by the river. —Hebrew, Rehoboth hannahar, Rehoboth-of-the-river, so called, perhaps, to distinguish it from Rehoboth-ir (Genesis 10:11). If the river is the Euphrates, this city was not on Edomite ground, and Saul probably reigned in Idumea by right of conquest.