How did ancient river systems create the gravel plains of Arabia?

The gravel plains of the Arabian Desert were formed by deposits left during the Pleistocene Epoch (approximately 2,600,000 to 11,700 years ago) by ancient river systems.1

These rivers, which are now represented by wadis such as Al-Rimah–Al-Bāṭin, Al-Sahbāʾ, and Dawāsir-Jawb, carried vast loads of sediment from the interior of the peninsula toward the Persian Gulf. As these ancient rivers reached the lower-lying areas, they formed extensive deltas that deposited the gravel and sediment that characterize these plains today. Notable examples include the Al-Dibdibah region, which was the delta of the Wadi Al-Rimah–Al-Bāṭin system, and the Al-Budūʿ Plain, which served as the delta for Wadi Al-Sahbāʾ. The remnants of these ancient river deltas are comparable in size to the delta of the Nile River.1