Basin form characteristics, causes and implications of the ten kongduis in the upper reaches of the Yellow River

The ten kongduis (the transliteration of ephemeral flood gullies in Mongolian) are one of the main sediment sources of the upper reaches of the sediment-laden Yellow River in China. The basin form characteristics and their causal factors and environmental effects of the kongduis are important knowledge for understanding the geomorphic processes and for managing the kongduis. In this study, we extracted the drainage networks and watersheds of the kongduis from the DEM, calculated the geometric and structural indices of the watersheds and networks, investigated relations of the gully density with surface material composition, vegetation coverage and topography, and the relations of the sediment yield with gully density and other geometric indices of watersheds, and associated the gully density with the evolution stage of landforms of the basins. The results reveal that the ten kongduis are 5th - 6th order streams and their watersheds are long and narrow with a circularity ratio of 0.16-0.26, an elongation ratio of 0.29-0.43 and a power of drainage area of 0.89-4.52 in the power function between mainstream length and drainage area. Yet, the available data do not support the impacts of the geometric indices of watersheds on the sediment yield in the ten kongduis, and the insignificant relation between the specific sediment yield and drainage area denies the existence of scale effect in sediment yield in the kongduis. In contrast, the gully density is found to be closely and positively related with the sediment yield. The hypsometric integrals of the upstream reaches of the ten kongduis and the relation between gully density and hypsometric integral suggest that the kongduis are in the mature stage of landform evolution and the west five kongduis are in the early mature stage with a potential increase trend of gully density and a concomitant escalation of sediment yield in future.