Keywords: 
• Yellow River /
• Yinchuan plain reach /
• channel bed /
• sedimentation rate /
• variation trend

Abstract: In the latest decades it has been confronted with the enhanced flood risk in the Yinchuan plain reach of the Yellow River and occurred several large floods have induced much economic losing. Many researches concluded that these flood disasters were resulted from the three large reservoir (Qingtongxia, Liujiaxia and Longyangxia) construction in the upper Yellow River because  of the operation of the reservoirs and the water derivation from the reservoirs and the main stream in the upper Yellow River induced the enhanced channel sedimentation rate in the studied river reach. However,  many previous studies in the world have showen that large dam or reservoir construction could induce the enhanced channel erosion or decreasing sedimentation rate in the dwonstream reach below the dam or reservoir. The change trends of channel sedimentation rates in longtime scale which includes anthropogenic dominant period (e.g. last 50 years) and natural dominant period (e.g. 1 000 years ago) could be used to reveal the influences of human activities on the channel change in the last decades. The aims of this study are to reveal the variation trend of the channel sedimentation rates in the Yinchuan plain reach of the Yellow River since 0.9 Ma B.P. and to evaluate whether the human activities have led to increase the channel sedimentation rates during the last decades. The methods used in this study are mainly the caculation of the average rate of channel sedimentation in geomophological scale according to the chronological analysis of a bore core with depth about 300 m, correlation analysis in different time periods since 0.9 Ma B.P., and the comparison of channel bed variation according to the 9 channel cross-sections in the river reach measured in the two different years of 1993 and 2001 that could be uesed to assess  the present channel sedimentation rate. The results indicate that the pulse function relationship between years and channel sedimentation rates obtained from the correlation analysis method can be used to estimate commendably the historical variation trend of the channel sedimentation rate in the study river reach in geomophological scale. The sedimentation rate has had an increase trend since 0.9 Ma B.P. as a whole, however, a decrease trend of channel sedimentation rates has occurred since 20 ka B.P. The peak value of 3.43 cm/a appeared in 20 ka B.P., it indictes that the maximum channel sedimentation rate was in 20 ka B.P. The average channel sedimentation rate since 0.04 Ma B.P. calculated from the pulse function is 3.16 cm/a. The present sedimentation rate estimated from this function is 0.51 cm/a which is close to the result of 0.3 cm/a caculated from the actual measurement of the channel cross-section variation. Obviously, the present channel sedimentation rate is far less the maximum sedimentation rate in the geomophological scale as well as the average channel sedimentation rate since 0.04 Ma B.P. The human activities, such as the water and soil conservation, large reservoir or dem constructions, and water derivation in the main stream of the upper Yellow River drainage basin, have decreased a lot of coarse sediment feeding to the Yinchuan plain reach of the upper Yellow river and decreased the channel sedimentation rate in the studied river reach. 