关键词: 黄河中游, 环境变迁, 水沙变化

Abstract: Recent years, geologists have discovered large areas of paleo aeolian sand existed in shifting sand dune, semi fixed and fixed sandy lands in Ordos Plateau. These proved that the desert was formed in prehistoric times. More proofs were recorded by 《Wei-su》 (History of the Northern Wei Dynasty):the Northern Wei emperor inspected the west in a A. D. 417, and hunting in "Great Desert" where might be today's Hobq Desert or Mu Us Desert by it's geography place. So the research results either from geology or historic geography suggest that the desert in Ordos Plateau had been existed in the Northern Wei or even before that time; the view of "man made sandy desert" taking farming after the Tang Dynasty as reason is untenable. After the eastern Han Dynasty, the quickly increased husbandry put great pressure and damage on the natural grass land, resulting from northern nomadic people massively moved into the middle reaches area of the Yellow River. According to 《Wei-su》, when one fight occurred in the place same as today's northern Shaanxi and Ih Ju League, the winner got livestock from enemy more than 2.03×107 head, average 125.5 head per km 2, and this figure near the level in early 1980' (130.9 head per km 2). Because of long term overstocking and backward herding manner, the grass land degraded seriously; Also, because the Wei Dynasty, Jin Dynasty and Sixteen Kingdoms periods were in the second cold dry period of recent 2000 years in China, so it is impossible to recover the damaged vegetation. In historic times the natural vegetation was ruined seriously in the middle reaches of the Yellow River, and added with great desert on Ordos Plateau and dry climate, heavy rain and flood was frequent there, During 18 years from a A. D. 468 to a A. D. 485, there were 9 times flood, average one time every two years. These proved that the consideration of "long term quiet" in middle Yellow River area after the Easter Han Dynasty is untenable.

Key words: Middle reaches area of the Yellow River, Environmental changes, Inundation