The Characteristics and Distribution of the Source Area of Aeolian Sand in the Valley of Lhasa River's Lower Reaches,Tibet,China

Based on IKONOS and Quickbird interpretation and field survey,the effects of human activities and characteristic of sand distribution,grain diameter of sand sources and vegetation in the valley of lower reaches of Lhasa River are discussed.The results indicate that valley sand sources distribute along river valley and its two sides,in patch and particle form from slope to bottom of valley,and have close interrelation with large,medium and small scale of wind field in space;There are three categories of accumulation substanceswhich are sand sources in the lower reaches of Lhasa River valley: river alluvium,flood depositand weathering substances of mountain rock.Among them,river alluviumis the most important sand source,grain diameter of sand source is mainly made of silver sand,extreme silver sand and sticky sand,and over 90% of whose grain size is less than 0.25 mm,and inclinedto produce wind erosion.Sand activity is the dominant factor affecting sand-nurtured vegetation.Especially the vegetation on semi-fixedand semi-movablesand duneis affected most seriously,species and coverage of vegetation can well reflect the stability of sand sources.Vegetation succession is developing in the tendency in favor of sand activities,which is one of a main driving factors and outcome of worsening sand activities.Though natural factors are the main cause of the sand activities in thisarea,human activitieswill further strengthen sand activities.