The source of the main channel of Wadi Halu is located approximately south of the summit of Jabal Sal, and southwest of the village and farming area of Sal. Although in
hydrology, when two rivers join it is customary to consider the one of lesser importance as
tributary, there are numerous rivers in which the tributary provides a greater flow, or has a greater length or surface area of its drainage basin than the main river. For example, the case of the
Mississippi River is well known, whose tributary (the
Missouri River) is, upstream of the confluence, longer and has a basin three times as extensive. In a much more modest dimension, it is something similar to what happens between Wadi Halu and Wadi Sal. from its headwaters, the Wadi Halu receives from the right the waters of its tributary the Wadi Sal, whose main course describes an arc long, from east to west, starting at its head. birth next to the village of Al Yafnah, also passing through the villages and extensive areas of cultivation on terraces or terraces, of Sal, Al Miʻli , with very thorny branches, and some
jujubes (jujubes / سدر / Sidr tree - Ziziphus spina-christi), species also very common in the
Hajar Mountains. In the last kilometers, before its mouth into the Wadi Naqab, next to a residential area of the town of Saih Al Hurf, the sides of the wadi are formed mainly by sedimentary materials that have been accumulating for thousands of years and have been eroded by the sporadic avenues, offering a kind of canal, with walls that rarely exceed in height, and have numerous cavities, similar to those that can be found in other wadis of similar morphology, used in the past, among other uses, as rudimentary refrigerators. Many of these cavities still conserve a part of the primitive
dry stone walls used to provide privacy, protect the rooms from the elements, and maintain a cool temperature to preserve food. == Floods ==