The river region is characterized by fertile mountain valleys and includes the Yarlung Tsangpo River (the upper courses of the Brahmaputra) and its major tributary, the Nyang River, the Salween, the Yangtze, the Mekong, and the Yellow River. The Yarlung Tsangpo Canyon, formed by a horseshoe bend in the river where it flows around Namcha Barwa, is the deepest and possibly longest canyon in the world. Among the mountains there are many narrow valleys. The valleys of Lhasa, Xigazê, Gyantse and the Brahmaputra are free from permafrost, covered with good soil and groves of trees, well irrigated, and richly cultivated.