过去关于水土保持措施影响的研究多侧重于减水减沙的影响,对泥沙组成影响的研究则相对较少。本文以大量的实测资料为依据,以河口至龙门间区域自然与水土保持条件为背景,将水保措施类型、河道水流泥沙条件与流域地貌条件结合考虑,借助统计分析方法对采用水保措施前后该区域（尤其是与黄河中游水土流失密切相关的黄土高原与沙砾丘陵区域）的泥沙粒径分布变化进行了探讨。分析表明,泥沙粒径分布变化极其复杂,对应于不同地貌类型条件的泥沙组成遵循不同的分布规律。黄土丘陵区河流中的悬移质泥沙级配服从正态分布规律,而沙砾丘陵区河流中的泥沙级配则服从皮Ⅲ型分布规律。当分布函数的类型确定后,泥沙粒径分布由该分布函数的几个参数共同决定,这些参数分别反映了泥沙从流域表层搬运到对应级别河流的过程中地貌形态及水土保持措施的影响。文中由多学科综合研究得到的结论,对黄河中游水土保持效益研究具有重要意义。

Special attention has been paid, in the present study, to the variations of suspended sediment size distributions in the tributaries as well as in the main stream within the middle reach of Yellow River with respect to regional water and soil conservations and the human activities. It is revealed that the conservations of water and soil in various catchments between Hekou and Longmen have significant impacts on the sediment size distributions in their corresponding rivers, and principally reduce median diameters of the grain groups. The sediment distribution functions in the study area are found to be controled by both the geomorphologic factors and the types of conservation activities in addition to the hydrological/hydraulic effects. For the hilly ground in Loess Plateau, the sediment size distributions in the rivers are fairly presented by the normal distribution with two important parameters, median grain diameter and the mean square deviation, which are altered by the specific mechanical practices and the landforms, respectively. For the sand gravel hilly ground, a Pearson Ⅲ type function is a good approximation for the sediment gradings, with three parameters altered by the conservation measures. The topic presented herein is of great value but is rarely approached in the previous studies. The applications of the findings are discussed in greater details in the companion paper.