Abstract: The sediment problems of the Yellow River have always been a hot topic in academia. The Yellow River has ten sediment-laden tributaries in Inner Mongolia, China, including Maobula, Buersetaigou, Heilaigou, Xiliugou, Hantaichuan, Haoqinghe, Hashenlachuan, Muhaerhe, Dongliugou and Husitaihe. This kind of tributary is called kongdui in Mongolian, meaning the flood ditch. They originate from the Ordos Plateau, flow through the Hobq Desert, and finally enter the Yellow River. Because of their special geographic environment, the ten kongduis are the principal sediment sources of the upper Yellow River, and their sediment output causes heavy sedimentation in the Inner-Mongolia reach. Like many tributaries on the Loess Plateau, which are the principal sediment sources of the Yellow River, the runoff and sediment load of the ten kongduis were reduced obviously in recent years. The changes in runoff and sediment load of the kongduis and their association with anthropogenic interference and climatic change should be studied for improving drainage basin management but have received less attention. In this paper, we applied simple linear regression, Mann-Kendall test method to systematically investigate the changes of annual precipitation, runoff, sediment load and flood processes of three kongduis(Maobula, Xiliugou and Hantaichuan) out of the ten tributaries using rainfall, hydrological and sediment data recorded in from 1958 to 2015. The results show that the annual runoff and sediment load of the Mabula and Xiliugou were declined significantly at the 95% confidence level, and those of the Hantaichuan had not a statistically significant declining trend but their means in the years after 2000 were noticeably lower than those before 2000. Besides, the water discharge and sediment concentration hydrographs of a flood in 2012(the recorded highest flood after 2000) were much lower than those of a flood in 1989(the recorded highest flood before 2000)although the difference between the rainfalls which produced the two floods was relatively small. The comparison of the mean annual runoff and the mean sediment load of the Maobula, Xiliugou and Hantaichuan from 1991 to 2015 with those in the period prior to 1990 showed a reduction of mean annual runoff in the range from 16.27% to 35.66% and a reduction of mean annual sediment load in the range from 60.85% to 76.67%. In contrast, the annual precipitation of the Xiliugou and Hantaichuan were increased by 9.52% and 3.00%, respectively, while the annual precipitation of the Maobula was decreased by 2.33% in the period from 1991 to 2015 than that in the period prior to 1990. Also, the variation of the high intensity rainfall for the kongduis was not significant. All these indicated that the main cause for the reduction of water discharge and sediment load of the tributaries is not the changes in precipitation, but the human activities. Since 1990s, lots of soil and water conservation practices have been carried out in the ten tributaries. In the period from 1989 to 2015, the barren land and farmland were decreased, the forest and grass land were increased, and the vegetation coverage was raised considerably. These changes in land use and vegetation coverage altered the process of precipitation-runoff-sediment yield, leading to the reducing of the runoff and sediment load in the ten tributaries.

Key words: runoff, sediment load, climate change, anthropogenic impact, the ten kongduis