Fluvial Responses To Late Quaternary Climate Change In The Shiyang River Drainage System,Western China

Fluvial process,which is controlled by climate change,tectonic activity and other variables,has played an important role in sculpting and shaping landforms.Climate,vegetation cover,and soil cohesion are the major variables that influence a river's discharge and sediment supply and hence control the channel aggradation and incision processes and fluvial styles.Therefore,fluvial response to climate change is one of the key subjects of interest for fluvial geomorphologists.As a drainage system located in arid western China,the Shiyang River,combined with considerable fluvial strata and landform information,provides an environmental context within which to investigate fluvial responses to late Quaternary climate change.Sedimentological analysis and optically stimulated luminescence(OSL)dating enabled us to reconstruct the processes and fluvial styles of three sedimentary sequences of the Shagou and Hongshui rivers in the Shiyang drainage system.Some important findings and conclusions are drawn as follows:(1)Basically,river behavior is controlled by the downstream gradient,discharge,and sediment supply.The upstream of the Shiyang drainage,namely the Jinta and Xiying Rivers,are dominated by gravel-bed braided rivers.However,the downstream of the Shiyang drainage,namely the truck Shiyang River,is characterized by sand-bed channels.Channel pattern shifting can be identified at Caiqi,with a meandering pattern in the upstream of Caiqi,and a braided one in the downstream of Caiqi.The Jinta and Xiying Rivers show a braided channel pattern which might be forced by a higher channel slope and the coarse bedload.Combing with a lower channel slope and fine bedload,the Wuwei-Caiqi reach is characterized by a meandering channel pattern,while the braided reach in the downstream of Caiqi might be a result of an increasing discharge.(2)Grain-size analyses show that the sediments of channel,floodplain and sand dunes can be identified by plots of mean grain size vs.sorting.The channel sediments are dominated by bedload with mean grain size of 147~336 ?m.The floodplain sediments are characterized by suspended load with mean grain size of 20~60 ?m.The sand dune samples are well-sorted with mean grain size of 109~266 ?m.(3)Our results present a variety of river behaviors during the late Quaternary in these areas.In the upstream Shiyang River,Zhangjiadazhuang(ZJDZ)profile of the Shagou was dominated by aggradation and a meandering channel pattern at 10.6~4.2 ka,while a noticeable channel incision occurred at ~4.2 ka followed by lateral channel migration.In the downstream Shiyang River,Datugou(DTG)profile of the Hongshui was an aggrading meandering river from 39.7 to 7.2 ka while channel incision occurred at 7.2 ka.Another downstream profile,Wudunwan(WDW)of the Hongshui was also characterized by aggradation from 22.4 to 4.8 ka;however,its channel pattern shifted from braided to meandering at ~13 ka.A discernable downcutting event occurred at ~4.8 ka,followed by three channel aggradation and incision episodes prior to 1.8 ka.The last 1.8 ka has been characterized by modern channel and floodplain development.(4)The fluvial processes and styles investigated have a close correlation with late Quaternary climate change in the Shiyang River drainage.During cold phases,the WDW reach was dominated by aggradation with a braided channel pattern.During warm phases,the rivers that we investigated were also characterized by aggradation but with meandering channel patterns.Channel incision events and changes of fluvial style occurred mainly during climate transitions.(5)An increase or decrease of discharge combined with a constant channel slope tends to result in different fluvial styles.From 22 to 13 ka,the WDW reach of the Hongshui River was dominated by a braided channel pattern,while the DTG reach was a meandering one.The meandering DTG at this time was probably caused by a lower bankfull discharge.While the WDW reach probably got much higher bankfull discharge that crossed the transition ‘threshold' and formed a braided channel.