| Two high resolution profiles of river sediments of Weihe River were found at Baijiaziu and Mafangtan, Gaoling County in Shaanxi province. Sidiments were sampled in two 350cm-deep pits that is located at Baijiaziu and Mafangtan. Detailed stratigraphic and pedological observations were made in these profiles during the field work. It indicats that the fluvial sedimentary sequence of the flood plain of Weihe River in this region is rather complete and continuous. The grain-size analysis carried out in laboratory has revealed the characteristics of the changes of Weihe River during last 1300 years. Based on multi-disciplinary analyses, including landform characteristics, grain-size and documents datum, the paper has attained the following important conclusions.1. The variation of coarseness and fineness of the component particles in Baijiaziu profile and Mafangtan profile is caused by river impetus of Weihe River, including runoff volume and velocity of flow, which is closely related with climatic factors in Guanzhong Basin, such as precipitation, temperature and rainfall. So grain-size can be used as climatic proxy to identify fluvial sedimentary layers and to reconstruct historical climatic changes in Guanzhong Basin.2. By the analysis of grain-size accumulative centigrade content, grain-size composition and grain-size parameters performed on samples in Baijiaziu profile and Mafangtan profile, nine stages of hydrological and climatic changes since the last 1300 years have been distinguished. The first, the third, the fifth, the seventh and the ninth layers of the stratigraphical sequence are finer while the second, the fourth, the sixth and the eighth are coarser. The coarser sedimentary layers reflect the period of warm and wet climate, during which the natural runoff of Weihe River violently rose, the speed of flow increased and the flood easily occurred; on the contrary, the finer sedimentary layers reflect the period of cool and dry climate, during which the natural runoff of Weihe River fell and the speed of flow slowed. These nine environmental changes were well reflected by the variations of coarseness and fineness of grain-sizeof fluvial sedimentary profiles near Dongweiqiao Site of Tang Dynasty.3. At vertical direction of Baijiaziu profile and Mafangtan profile, the grain-size distributional coarse and fine rhythms vary very pronouncedly with the depth of the stratigraphical sequenc. Md, Mz, o , Sk and Kg of the sedimentary sequence separately constitute a commulative between peak and valley values. The grain-size distributional coarse and fine rhythms clearly show the well-regulated fluctuation of the climatic changes that result in the changes of river impetus of Weihe River. Taking the eigenvalue Φ 50 as a boundary, theΦ75, Φ 84 and Φ 95 above the eigenvalue have wide flounce range and high sensitivity responding to the sedimentary sequence, which may be called "active eigenvalues" . In contrast with them, those below the above-cited eigenvalue, i.e., Φ5, Φ16 and Φ25, which have relatively narrow flounce range and low sensitivity, are called "inert eigenvalues" .4. The comparison of the component particles of Baijiaziu profile and Mafangtan profile with that of modern flood plain substance of Weihe River indicates that the river impetus of Weihe River has gradually been getting weaker, which reveals that it has gradually been getting dryer since the last 1300 years. These inferences are generally in aggrement with what was previously concluded by documents datum and archaeology information. The trend of drying up of Weihe River since the last 1300 years is the regional response of this environmental sensitive zone to the globe warming.5. In the course of north moving, Weihe River downcut clearly and the riverbed of Weihe River descended about 6.3m because of the ascending of the new conformation. Meanwhile, the flood peak fell off ceaselessly, witch indicates that the new conformation descended greatly. The thickness of flood plain facies subs...
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