The Tang River (党河 in Chinese, Dang River or Danghe in the modern Chinese transliteration) which is so important as a navigation landmark in Biggles in the Gobi is an actual river in the Gansu province of Western China where most of the events in the book take place.
The Dang River is an inland river i.e.it doesn't reach the sea. Instead it drains into an inland basin where it peters out. The river rises in the Qilian Mountains which form the boundary between Gansu province and Qinghai province to the south. Fed mostly by glacial meltwaters, the river flows northwest and then bends to turn north until it reaches the city of Dunhuang which also features in the book. The river is a major source of water for the city.
In the book, Nan-hu, Biggles' destination, where the missionaries were believed to be in hiding, was located some 20 miles south of Tunhwang (now Dunhuang) along the river. Nan-hu, a set of artificial caves carved out of a set of cliffs rising from the river is a fictional name but co-incidentally, the Western Thousand-Buddha Caves is actually in approximately the same location and is a set of artificial caves with exactly the same features as Nan-hu.
In 1975, a 46m high dam was constructed upstream of the Nan-hu location to create the Dang River Reservoir (or Danghe Reservoir). The reservoir, with a capacity of some 15 million cubic metres supplies water to the city of Dunhuang and for agriculture in the region.