A river and wady of eastern Palestine, the modern Wady Mojib (or Wady el-Mojib). The name means perhaps "noisy," a term which well describes the latter part of the course of the river. Its length is about 45 miles, from its rise in the desert to its entrance into the Dead Sea. It spreads out to a breadth of 100 feet here and there, but for the most part is narrow; and though low in summer, in the winter season it is in places 8 or 10 feet deep. It runs at first northwesterly, but afterward its course becomes westerly. Its striking feature is the steepness and narrowness of the ravine through which it passes shortly before it empties into the lake, opposite Engedi. Between the lofty limestone hills, which cause this precipitous descent, and the lake, the river expands into a shallow estuary nearly 100 feet wide.
The Arnon has always been an important boundary-line. Before the Hebrew period it separated, for a time at least, the Moabites from the Amorites (Num. xxi. 13, 26; Deut. iii. 8; Judges xi. 18). After the Hebrew settlement it divided, theoretically at least, Moab from the tribes of Reuben and Gad (Deut. iii. 12, 16). But in fact Moab lay as much to the north as it did to the south of the Arnon. To the north, for example, were Aroer, Dibon, Medeba, and other Moabite towns. Even under Omri and Ahab, who held part of the Moabite territory, Israel did not hold sway farther south than Ataroth, about ten miles north of the Arnon. Mesha in his inscription (Moabite Stone, line 10) says that the Gadites (not the Reubenites) formerly occupied Ataroth, whence he in turn expelled the people of Israel. He mentions (line 26) his having constructed a road along the Arnon. The ancient importance of the river and of the towns in its neighborhood is attested by the numerous ruins of bridges, forts, and buildings found upon or near it. Its fords are alluded to by Isaiah (xvi. 2). Its "heights," crowned with the castles of chiefs, were also celebrated in verse (Num. xxi. 28).
The Haggadah tells the following story of a miracle witnessed at the Arnon, which seems to be alluded to in the Bible (Num. xxi. 14, 15). The mountains bordering on the Arnon consist of two lofty ranges, with a valley, seven miles wide, between them. When on the way to the promised land, the Israelites, after having crossed the first range, prepared to cross the second, the Amorites hid in the caves, intending to attack the unsuspecting travelers. But the Ark of the Covenant, which preceded the Israelites, caused the heights to sink and the valley to rise, with the result that the concealed Amorites were crushed in the caves. The miracle would have been unnoticed by the Israelites, had not God caused the well which accompanied them to throw up portions of the corpses. Then it was that all Israel sang the Song of the Well (Num. xxi. 17 et seq.). In commemoration of this miracle the Rabbis decided that a special benediction be uttered upon seeing the Arnon (Ber. 54a et seq. Num. R. xix. 25; Tan., Ḥuḳḳat., xx.).