Abstract: Ten kongduis(the transliteration of ephemeral flood gully in Mongolian), is ten tributaries on the south bank of the upper Yellow River between Bayangol and Hekou towns. The ten kongduis are one of the principal sediment sources to the upper Yellow River. The Maobula River, one of the ten kongduis, lies at the extreme west. With its special geographical settings, the Maobula River is dominated by wind and water erosion and hyper- concentrated flows. As a tributary from the desert, a large number of coarse sediment from the Maobula River has been transported to the Yellow River with the hyper-concentrated flows. These coarse sediment deposits in the Yellow River, resulting in the elevation of riverbed and even blocking the river for days sometimes, causing serious sediment disasters and huge economic losses. However, previous studies mainly focus on the transporting characteristics of sediment of the Maobula River, and the grain-size distribution characteristics of sediment in the catchment were rarely reported. Grain size is a key factor impacting the sediment transport and river channel changes, meanwhile, it is one of the main factors controlling the pollutant distribution in sediment, especially for the adsorption and release of heavy metals. Therefore, in this paper, the grain size distribution characteristics of sediments in the Maobula River channel and influencing factors are investigated using the moment method for calculating grain-size parameters and analysis of variance. The grain size distribution characteristics are very important to understand the change of sedimentary environment of the study area, implement the soil and water conservation measures in the wind and water erosion area and reduce coarse sediment transported to the Yellow River. Along the Maobula River, seven typical sampling points were set, and analysis of grain-size fractions was carried out by using Mastersizer 2000. The results show as follows: (1)The sand grains are the main component of the deposits on average, and the silt and clay grains account for only 14.62% and 1.61%, respectively. The content of sand presents a decreasing trend from the upstream to downstream.(2)Average grain size ranges in 1.17-5.56 Φ with an average of 2.62 Φ, all falling in the sand grain size range. Overall, the average grain size decreases downstream from the gully head; the standard deviation of all sediment samples is over 1.0, indicating a bad sorting degree. Deposits collected at seven sites have a positively skewed grain size frequency distribution curve with a sharp kurtosis ranging from 6.99 to 15.63. The average grain-size and sorting coefficient are positively correlated, and skewness and kurtosis are also positively correlated.(3)The main transporting form of sediment in channel of the Maobula River is bed load. The results of this paper reflect that the sediment in the Maobula River channel has the characteristics of sediment being sorted in flows and showed that low flows can not bring coarse sediment into the Yellow River.(4)The particle composition of sediment sources can be seen as an internal factor influencing spatial discrepancy of sediment particle composition and the hydrodynamic as an external factor. Through ANOVA, the geomorphic condition was found to be the main factor influencing the grain size, and the coarse sediment mainly came from the hilly and gully region in the upper steam. Therefore, the key of reducing coarse sediment from the Maobula River transporting to the Yellow River is to strengthen soil and water conservation of the upstream slopes and gullies.

Key words: Maobula River, river bed deposits, particle size distribution, geomorphic condition, hydrodynamic