As branches of the Nile River descend from the highlands of East Africa, they join in a single course and pass through the land of Nubia, in what is now northern Sudan and southern Egypt. The river has always provided life in this arid region as a source of water, food, and transport. It flows through areas known as cataracts, traditionally numbered from north to south, where the river valley narrows and rocky outcrops define islands, rapids, and waterfalls. The Nile also traverses broad plains that provide a basis for agriculture as well as for concentrations of population, wealth, and power. The savannas and deserts on either side of the river are integral to settlement, supplying raw materials including gold as well as areas for herding and hunting.