The Republic of Yemen is located on the southwestern coast of the Arabian peninsula, the region once possessed by the ancient southern Arabian kingdoms that occupied the mouths of large wadis (valleys) between mountains and desert. The first-millennium-b.c. kingdom of Saba sprang up in the dry delta of the Wadi Dhana that divides the Balak Hills. In the eighth century b.c., at the height of their prosperity, the Sabaeans had established colonies along both sea and land trade routes to Israel, and they dominated the region. Their capital, Marib, among the wealthiest cities of ancient Arabia, stood 107 miles (172 kilometers) east of Sanaa, the capital of modern Yemen. It is generally agreed that artificial irrigation was practiced near ancient Marib as early as the middle of the third millennium b.c. About 2,000 years later a dam was built to harness the biannual floods and systematic irrigation was introduced. Some scholars believe that the Marib Dam was the greatest technical structure of antiquity.
Around 685 b.c., under King Karibil Watar, Saba enlarged its borders. Territories were conquered in the southwest of the peninsula Ausan in the south was defeated and Sabaean rule extended northwest as far as Nagran. In the second half of the sixth century b.c., two kings successively built the Marib Dam near the mouth of the Wadi Dhana, the largest water course from the Yemeni uplands. By impounding water during the two rainy seasons, the dam provided irrigation for some 25 square miles (65 square kilometers) of fields and gardens. Replenished and enriched by sedimentary deposits, this agricultural land supported a population estimated to be about 30,000.
The first dam was a simple earth structure, 1,900 feet (580 meters) long and probably only 13 feet (4 meters) high, built between rocks on the south side of the wadi and a rock shelf on the north. Its location a little downstream of the wadis narrowest point permitted space for a natural spillway and sluices. Around 500 b.c. a second 23-foot-high (7-meter) earth dam was built. It was triangular in section both faces sloped at 45 degrees and the upstream side was faced with stone set in mortar. The final form of the Marib Dam was not built by the Sabaeans.
Late in the second century b.c. the Himyarites, a tribe from the extreme southwest of Arabia, established their capital at Dhafar and gradually absorbed the Sabaean kingdom, gaining control of South Arabia. They undertook the next major reconstruction of the Marib Dam, building a new 46-foot-high (14-meter), 2,350-foot-long (720-meter), stone-faced earthen wall, incorporating sophisticated hydraulic systems. It was nearly 200 feet (60 meters) thick at the base, built on a stone foundation, and created a lake that was probably 1.5 square miles (4 square kilometers) in area. At each end of the wall there were sluices, constructed with what has been described as the finest ancient masonry