Fluvial Geomorphic Evolution And Terraces Distribution At The Upper Yellow River,Jingyuan Section

River terraces,basin shape,drainage density,and fluvial channel of a watershed are influenced by a variety of geological and climatic variables.The landform evolution of Baiyin City,Gansu province controlled by the activity of Haiyuan Fault and the degradation/erosion of the Yellow River.The Haiyuan Fault controlled the geological structure of this region,while the Yellow River controlled the local base level and left a staircase of terraces.Researches had previously emphasized tectonic impacts on fluvial system in tectonically active regions,while the effects of lithology and local base level comparatively got little attention.Here,we investigated the fluvial landform at different spatial scales,focusing knickpoints and channel network reorganization,from an area affected by the Haiyuan Fault in the northeastern Tibetan Plateau.The geomorphic indices,i.e.steepness index,valley width-to-height ratio,chi anomalies were calculated and investigated and analyzed how the evolution of fluvial landform response to tectonics,lithology,and local base level.In order to investigate the Yellow River evolution in this place,we also explored the staircase strath terraces at the Wufo basin.Based on the method of soil stratigraphy and magnetostratigraphy,we analyze the loesspaleosol sequence which overlying the gravel layer of the high terrace.The results show that:Two regional radiative drainages formed around Laohu and Hasi mountains.Interior of the radiative drainage,tributaries from the southeast side of Laohu mountain experienced near 180° direction change.We interpret this as the gradual drainage capture,which originated from the height difference(?190 m)of the local base level.Some tributaries from Hasi mountain show alternating lithological-controlled gorges and the broad valleys.Besides,tectonic uplift and the lowering of base level(from the incision of the Yellow River)triggered an autogenic positive feedback transition from the parallel to dendritic drainage patterns.These observations suggest the base level and lithology play a crucial role in landscape evolution,even in a tectonically active region.At least 9 strath terraces were identified at the Wufo basin.The loess which overlying the T8 gravel layer,were around 43 m,and record 13 cold-warm cycles and develop 13 loess-paleosol sequences.The magnetostratigraphic results show that the loess-paleosol sequence includes two positive and negative polarities.So,the T8 terrace abandoned around 1.2 Ma.Combines with previous research and the age of T8 terrace at the Wufo basin,we think that,the Yellow River has cut-through the LanzhouSanmenxia section and flows east to the Bohai bay at 1.2 Ma.While the Jilantai-Hetao ancient lake was connected with Yellow River as a barrier lake,not a tail-end lake.