PALEOENVIRONMENT IN THE LOWER REACHES OF HEIHE, INNER MONGOLIA, NORTH CHINA. MIGRATION OF HEIHE AND JUYAN LAKES

1. INTRODUCTION On the north slope of Qilian Mountains, broad alluvial fans extend toward northernmost end of the Heihe drainage basin. Juyan terminal lakes, such as Sogo and Gashun lakes are situated in desert environments. Recently they were dried up, excluding temporary supply of water. In addition, another large lake, the Old Juyan lake existed in the east. It has been dried up in the historical time. Topography and surface geology of Juyan and Old Juyan lake areas were investigated in relation to historical evidence, as a part of RIHN Oasis project.

2. OLD JUYAN LAKE SEDIMENTS AND THE SHORELINES By the shore of Tian-e-hu, a small lake at NW end of Old Juyan lake, 1.5 to more than 10 m thick lake sediments are exposed and partly eroded, showing yardan topographies. Mischke et al. (2002) investigated this sediment by using ostracoda, and indicated that the lake existed at least 2700 to 5400 cal.y.BP. In our study, one of yardan mounds by the shore of 2002 was investigated and a radiocarbon age shows the lake already existed at 4600 cal.y.BP. Surrounding the Old Juyan lake including Tian-e-hu, a number of gravel bars are distributed and preserved well. They are good evidences of the former lake shorelines. 23 gravel bars were distinguished along the 2002 survey line. A raidocarbon age of the 5th gravel bar from the highest is about 3300 cal.y.BP. Corona satellite photos show the distribution and alignment of the lake shorelines well around the areas of Old Juyan lake. It suggests the gravel bars may be the evidence of a large lake covering all of Old Juyan lake area.

3.MIGRATION OF HEIHE AND JUYAN LAKES On the southwest of Old Juyan lake, some historical sites ranging from 2000 to 650 y.BP are located. It suggests that the Old Juyan lake existed upto the historical ages. At present, a sand dune zone separates Old Juyan lake and historical sites. Therefore the course of Heihe was changed from NE to NNE probably by advance of the sand dune zone. The river water must cut and erode existed higher sand dune zones, then flow into the Juyan lake basin. Along the north course, that of the present east river of Heihe, there are some outcrops composed of a number of flood deposits interbedded with thin weak soils. Near the lowest horizon, a thicker soil underlies the flood deposits. Its age is about 600 cal.y.BP. This age may support the migration model of Heihe and Juyan lakes.