Wadi Al-Rummah or ar-Rummah is one of the Arabian Peninsula's longest river valleys, at a length of almost 600 km (370 mi). Now mostly dry and partly blocked by encroaching sand dunes, the wadi arises near Medina at Al-Abyad Mountain (The White Mountain). It heads towards the north-east, connecting to several smaller wadis, like Mohallani Wadi and Murghala Wadi to the north and Jifn Wadi and Jarir Wadi to the south. It ends at Thuayrat Dunes of the ad-Dahna Desert in Al-Qassim Province.
The wadi then sinks beneath the sand dunes, where it is called Mistewy Wadi. It emerges on the other side of the desert as Wadi Al-Batin (approx. 425 km), which continues towards the north-east and forms the western boundary of Kuwait. It empties finally into the Persian Gulf.
The valley is wide, for it was once a major river valley. According to Dr. Abdullah Al-Musnad from the University of Qassim, about 10,000 years ago it was a river flowing from Medina to the Persian Gulf, with a total length of 1200 km. Periods of drought and the movement of sand at Althwairat and Dahna led to the course of the valley being cut into three parts: Wadi Al-Rummah (the longest, at 600 km), Wadi Aloddi (45 km), and Wadi Al-Batin (450 km). Geological studies show that Wadi Al-Rummah flows at full capacity about three times every 100 years. It flowed most recently in 1945, 1982, 1987, 2004 and 2008. In 1818, the river valley was flooded for 40 days, in 1838 for 22 days, in 1987 and 2008 for 17 days. In 1838 the wadi overflowed, creating a 200-square-mile (520 km2) lake that persisted for two years and attracted water birds rarely seen in the valley.