YARMUK (Heb. יַרְמוּךְ), a confluent of the Jordan River on its east side. The Yarmuk is the second largest river in Ereẓ Israel (after the Jordan) in volume, and the third largest in length (after the Jordan and Naḥal Paran); it is the largest river in Jordan. The etymology and meaning of the name are unknown. The Yarmuk is first mentioned in Hebrew in the Mishnah: "The waters of the Jordan and the Yarmuk are invalid because they are mixed waters" (Par. 8:10), i.e., it was forbidden to use them for sprinkling in cases of uncleanness, for which purpose only running water is permitted (Num. 19:17). According to *Estori ha-Parḥi, the mixed waters in the Mishnah refer to the waters of Ḥammat-Gader, which empty into the Yarmuk (Kaftor va-Feraḥ, 7, ed. Luncz, p. 125) – thus the Mishnah considered the Yarmuk as the river starting at Ḥammat-Gader and its continuation below. In the Babylonian Talmud (BB 74b) and Midrash Tehillim (to 24), on the other hand, the entire course of the river is called the Yarmuk, since it is enumerated as one of the four rivers comprising the border of Ereẓ Israel. The Yarmuk appears in variant spellings in Greek and Latin: Pliny the Elder, in the time of the destruction of the Second Temple, mentions it as Hieromices (not Hieromax) and it appears as such on coins of Gadara; on the Peutinger Map it appears as Heromicas. Byzantine writers of the 8th–11th centuries refer to it as Hiermochthas or Thos. In Arabic the lower course is called Nahr al-Yarmuk and the middle course is called Shari ʿ at al-Munaḍira.
On maps the upper course coincides with Wadi Zaydī, the longest intermittent stream joining the Yarmuk. Wadi Zaydī rises in the heights of Mt. Bashan and receives water from ephemeral streams descending from Tell al-Jaynā ʾ and Tell al-Kulayb, for about 12½ mi. (20 km.). In the mountains it contains water all year long. In the tableland it passes by Bozrah and Edrei, and after this point its continuation, called Wadi al-Madān in Arabic, contains water most of the year and often all year long. About 9 mi. (15 km.) northwest of Edrei the Madān is joined by Wadi al-Dhahab, which also descends from the Bashan, and 2 mi. (3 km.) after this junction a tributary with abundant and perennial water – Wadi al-Bajja – empties into the Dhahab. Wadi al-Bajja issues from a small lake and springs in Muzayīb. From this junction and below, the Yarmuk is a perennial river even in years of drought. After Edrei the bed of the Yarmuk deepens and after passing the mouth of the Bajja it becomes a deep gorge, which is much lower than the plateau on both its sides.
The tributaries to the east and west have not cut courses as deep as the Yarmuk and the smaller they are the higher is the level from which they fall to the bed of the Yarmuk. Waterfalls are thus created – in the small tributaries, right on the slope of the Yarmuk, and in the large tributaries, which have hollowed out channels as deep as the main bed, at some distance away. The waterfalls are especially numerous during heavy rains, when water also rushes down the ephemeral streams. The first large permanent waterfall is situated near Zīzūn north of the Yarmuk. About 3 mi. (6 km.) west of Zīzūn the Yarmuk receives the al-Shallāla River, which flows from the south from northern Gilead near the Jabbok; it is a perennial stream for the last 12½ mi. (20 km.) of its course. Then the al-ʿAlān River flows from the Bashan plateau and joins Wadi al-Iḥrayr (or Ḥarīr) near its mouth. In Arabic the ʿAānl is called "nahr," i.e., a perennial stream, but this is true for only about 9 mi. (15 km.) of its length. The Iḥrayr rises in the northern Bashan tableland near the large village of Sanamayn, and the network of its tributaries also extends through the northern part of Mt. Bashan. Because of its large drainage area, huge quantities of water rush down during torrential rains and contribute much to the flooding of the Yarmuk.
The last large tributary of the Yarmuk is Nahr al-Ruqqād, which rises in northern Golan near Tell al-Shaykha and passes through the eastern part of the Golan. From the mouth of Wadi Shallāla to Ḥammat-Gader, the Yarmuk plain forms an arc to the north whose chord is about 12 mi. (19 km.) long, and when the lower part of the Shallāla is added, a semicircle is formed with a diameter of about 13½ mi. (22 km.). Inside the arc several small streams from the Gilead empty into the Yarmuk. From Ḥammat-Gader the Yarmuk flows in an easterly direction, and when it enters the Jordan Valley, it turns southeast to follow the southern slope of the valley. As long as the Yarmuk is in a gorge, its bed is 56 to 63 ft. (17 to 19 m.) wide and only a few small sections are not affected during its flooding. In the Jordan Valley the bed at first is only slightly lower than its banks but after several hundred meters the river bed cuts deeply into the soft marl soil.
A famous battle between the Byzantines and the Arabs was fought on the banks of the Yarmuk on Aug. 20, 636. Outflanked by the Muslims from the northeast and blinded by a desert wind from the sandy south, the Byzantine army perished in the ravines of the river gorge; this battle decided the fate of Syria in favor of the Muslims. In memory of this victory, the "Liberation Army" of Fawzi al-Kaukji in 1947 called itself the "Army of the Yarmuk"; it was defeated and dispersed in 1948 near *Mishmar ha-Emek.
To utilize the Yarmuk's water for irrigation by gravitation, it must be diverted before it leaves the gorge either into a canal extending along the eastern end of the Jordan Valley or into a high aqueduct. The former method was used by the Jordanians in 1960–62 and the latter by Jews in Jordan Valley settlements. In 1908 a railway line was installed through the Yarmuk Valley to connect Haifa with the Hejaz railway (now operating from Damascus to Ma'on in Edom), but in 1948 this linkup with Haifa was cut and the railway's importance ceased. From 1920 the Yarmuk formed the boundary between Syria and Jordan. Its name regularly crops in talks about a regional peace agreement in the Middle East, due to its importance as water source for Israel, Syria, and Jordan.