Holocene terraces along most rivers in the NW China were well developed,while the process and mechanism about the Holocene terrace is poorly known.In the eastern Qilianshan Mountain (between 101 °30′E to 103°30′E and 37°00′N to 38°10′N),near Wuwei City,Holocene terrace is usually composed by 1 ～3 flights of sub-terraces,while this phenomenon is scarcely found with Pleistocene terraces.In order to disclose the mechanism of the formation the sub-terraces,we investigated the Holocene terraces in 13 river valley transections along 4 rivers(the Xiying,Jinta,Huangyang,and Gulang rivers) in the eastern Qilianshan Mountain.The depositing stratigraphy with each terrace was observed in detail and terrace age was accurately determined by 14C dating method in the relative sediment of the Holocene terrace.Except the Xiying River,2 ～ 3 flights of Holocene sub-terraces were developed along other 3 rivers,especially along the Jinta River,the sub-terraces are distinct.Within each terrace stratigraphy,Holocene paleosol was usually developed in the overlying loess or colluvial sediment above the fluvial gravel layer.Analysis results of terrace ages indicate that river incisions mainly cumulated on the period of 7.5 ～ 3.0ka B.P.Based on the comparison between river incision ages and climatic records,we find that during the periods (5.9 ～7.2ka B.P.,4.2 ～5.2 ka B.P.and 2.4 ～ 3.3ka B.P.) of river incision,the climate was relative wet in the study area.This evidence may suggest that river incisions happened while the discharge increases and the sediment supply decreases according to increasing precipitation and better vegetation cover.The multiple incision periods in Holocene also suggest that one regular river incision(like Pleistocene terraces)is completed by multiple short-lived incisions and aggradations,and due to the local river valley situation,this process can be documented by small terraces staircases,or it will be erased by side erosion.Otherwise,different rivers with different sizes may have different responses to same climate changes that large rivers are sensitive to long-term climatic changes,while it is not sensitive to short-term climatic changes.%全新世期间西北地区河流普遍发育多级河流阶地,而关于其形成原因和过程仍不甚明了.我们对祁连山东段13个河流横断面的全新世阶地进行了详细的野外考察,并利用14C测年方法对阶地的形成年代进行了限定.结果显示,全新世河流下切主要发生在7.5～3.0kaB.P.,通过对比河流下切年代与区域气候变化,发现河流下切集中的3个时段(5.9 ～7.2ka B.P.,4.2 ～5.2ka B.P.和2.4～3.3ka B.P.)对应于区域气候较湿润时期,相对较高的径流量与植被覆盖较好、沉积物供给较低的条件下,河流发生下切.全新世河流的多次下切指示一次完整的河流下切过程可能是通过多次下切与短暂的堆积过程交替而完成的,但由于局地状况的差异,这种过程可能被高差较小的阶地记录下来,也可能被河流侧蚀抹掉.不同河流对气候响应的敏感性有差异,较大河流对短期的气候变化响应不敏感,主要受长尺度气候变化的控制作用.
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