Bedrock River Incision and Loess Deposition Response to Basin Connection of The Sanmen Gorge, The Yellow River, North China

Basin connection drives the critical transition of fluvial system, and thereby induces rapid incision within the river course and changes the compositions of the associated deposit. However, absence of continuous sediment record in most large fluvial system make it difficult to reconstruct the long-term process of basin connection and its subsequently response. Here we present the longitudinal profile of the Sanmen Gorge, the Yellow River, where extensive fluvial terraces are in well preserved by diverse loess-paleosol sequence with previous dating, which shows the connection of upstream basin via the knick-point retreat within the gorge induced rapid incision along the downstream river course since late mid-Pleistocene. We apply the trace elements and major elements composition for comparisons among the loess deposit in the floodplain area downstream the gorge (the MS loess), the terrace sediment along the gorge and the typical loess in China Loess Plateau (the LC loess), which suggests the excessive deposit compensation on the floodplain due to the great upstream sediment flux after basin connection became an extra supply for the MS loess since late mid-Pleistocene, substantially increased the thickness, the accumulation rate and grain-size comparing with the LC loess. We also present heavy mineral indexes comparison of terrace sediment, which show proximal source before but distal supply after the basin connection. These observations provide rare continued sediment record for the long-term fluvial evolution of large river, suggesting that critical transition of fluvial system positively correlated with upstream basin connection and subsequently great change on drainage area and discharge.