if cut off entirely from its feeding source. In 2008 the annual evaporation was reported as 2,901 mm. Historically there were periods when the area was more favorable to farming and settlement than today. Studies showed that the area experienced seven major climate changes since the end of the Pleistocene. Lop Nor There are numerous indications that suggest the presence of an extensive lake in this region which is now completely desiccated. These indications include salt-stained depressions of a lacustrine appearance; traces of former lacustrine shorelines, more or less parallel and concentric; the presence in places of vast quantities of fresh water mollusc shells (species of Lymnaea and Planorbis); the existence of belts of dead poplars; patches of dead tamarisks and extensive beds of withered reeds, all of these are always on top of the yardangs, never in the wind-etched furrows. In Hanshu (the Book of Han, a history of China completed in 111), where it was called Puchang Hai (蒲昌海), the lake was suggested to be of a great size, with a dimension of 300 to 400 li (roughly 120–160 km, 75–100 mi.) in length and breadth. It was also called Yan Ze (鹽澤) in Shiji, which means "salt marsh", indicating that the lake was salty. The lake had already shrunk considerably by the Qing dynasty. It had shifted its location to Kara-Koshun by the latter half of the nineteenth century, then back again to Lop Nor in 1921 through human intervention. The building of dams by Chinese garrisons in the twentieth century blocked the water from the rivers feeding in to Lop Nor and it is now primarily salt flats. The dried-up Lop Nor basin is covered with a salt crust from thick. Flora and fauna Natural vegetation is sparse in the region and poor in the number of species. A scientific expedition to the Lop Nor region in 1979-1982 collected only 36 species of plants, belonging to 13 families (mainly Chenopodiaceae and Compositae) and 26 genera. The expedition also collected only 127 species of animals (23 mammals, 91 birds, 7 reptiles, and 1 amphibian). Archaeologist Sven Hedin who travelled in the region in the late nineteenth as well as the twentieth century was able to travel by boat up the rivers to the lake and saw a multitude of wildlife. However, many wild animals, such as tiger, wolf and wild hog which had been found by former