| The problem of soil erosion in Loess Plateau has become one of the environmental problems for the attention of the whole society. A small basin in Loess Plateau is the key to study the watershed sediment yield laws and formulate control measures. Yet, the natural formed dam or artificial dam system in small basins which hosts many information about sedimentary environment by quickly depositing the retained sediment still need developing. So, choosing natural formed dam as the object of the study and finding out the regulation of sediment generation which controls basins, the characteristics of erosive environment and the effects which human activities have on the basins can provide strong data support and scientific basis for the further study of exploring the regional erosion laws on Loess Plateau and harnessing regional soil and water loss effectively.This paper takes the small watershed natural ancient poly marsh of Huangtuwa in Zizhou of Shaanxi Province as the research object. Through field investigation, fundamental data collection and the experimental analysis of the samples'particle size and others. The soil profiles were selected according to the horizontal and vertical section in the reed edge and the Wuchushan channel, and 6 soil profiles were collected. Then, the characteristics of grain-size to the 2010 samples of the six were measured. The characteristics of the spatial distribution laws of particle size of sediments in the poly marsh were analyzed. And according to the characteristics, the typical sediment layers were divided. Combined with precipitation data in the watershed basin in Huangtuwa from 1953 to 2014 to the dating techniques of 137Cs, the time coordinate of typical sediment layers was build, the typical cyclothem of silt deposition in Wuchushan channel was estimated and the sedimentary environment in the basin was divided initial. At the same time, we elaborate the impact of human activities on the sediment deposition, figure out the temporal and spatial distribution laws of deposited sediment in the basin and finally reveal the deposition mechanism of the natural ancient poly marsh. The conclusions are as follows:(1) Based on the characteristics analysis of natural poly marsh grain-size in Huangtuwa, we can find that the soil particle size is fine in the whole area and mainly of silt content. According to the rules of sediment deposition cycles, the typical cycle layer of sediments of six soil profiles were analyzed. The result showed that the correspondence of three soil profiles(W?T?Y) layer cycle in the cross section were poor, and the conclusion showed that human activities have great effect on silt dam sediment layer. While the particle size of three typical cycle layer of sediments(a?b?c) in four soil profiles(V?U?T?P) in the vertical section became finer when the profile closer distance from the center of dam, the particle size became finer and the sedimentary bedding became more obvious. According to the grain size frequency curves of these six soil profiles, the most forms of frequency curves were a single peak. And this conclusion showed that Huangtuwa region size variation was mainly affected by precipitation, less wind effect, the sedimentation of a single sedimentary environment is stable.(2) Based on 137Cs dating techniques and the precipitation data of the region, the time of every sedimentary layer was inferred. The layer formed in 1994, b layer formed in 1959. According to the corresponding relationship between rainstorm and coarse particles, combined with historical records of rainfall data to infer the c layer was formed in 1944 near. The three cycle thickness is relatively large, and it mainly by heavy rain or continuous large erosion caused by rainfall.(3) The volume-weight of 32 soil samples in the channel was tested. The deviation was 3.47%. And the sediment volumes of a, c layers were 3976.346m~3 and 5444.190m~3 respectively, which were calculated according to the average soil volume-weight,1.45 g/cm~3. Finally, the sediment deposition was also estimated. The calculation of sediment deposition in 1994(a layer),1944(c layer) were 5765.702t and 7894.076t, respectively.(4) According to the change law of the dual structure in the deposited sediment of the profiles and the growth characteristics of reed, we can infer that the deposited sediment of the P profile was developed in the deposited environment of normal poly marsh. After comparing the deposited sediment variation laws of soil profiles in the vertical section, we found that the three profiles (T,U,V) wasn't developed in the deposited environment of normal poly marsh. Combined with grain-size variation laws and field work of corn growing states in the channel inlet of Wuchushan channel, we preliminary inferred that the boundary of the normal poly marsh and the abnormal poly marsh was nearby the channel inlet of Wuchushan channel. From the boundary started, the deposited sediment type forwards the natural dam is the normal poly marsh, while towards the channel inlet, the type is the abnormal poly marsh.(5) According to the analysis of grain-size distribution in the sedimentary cycles of soil profiles in the horizontal and vertical sections, we can found that the human activities have great impact on the sedimentary environment in the channel of Huangtuwa basin. When people do some farming activities, the surface soil was affected, and so the thickness and integrity of sedimentary layer were destructed. And this effect also increased the difficulty of sediment accumulation calculation. |
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