Investigation On Braided River Geomorphology In Yarlung-Zangbo River

Braided river is one of the basic river types.Its geomorphological characteristics and evolution are complex and have been widely noticed.In recent years,researchers conducted high-resolution and high-precision quantitative analysis on prototype braided rivers based on new observation techniques such as satellite remote sensing and global positioning systems,and made some progresses.However,the existing researches are mostly focused on the morphological change and sediment erosion and deposition of local geomorphic units in small and medium-sized braided rivers.Overall geomorphology of large braided rivers has not been investigated systematically yet.The Yarlung-Zangbo River?hereinafter referred to as YJ River?originates from Tibet,China.Typical braided river forms in its wide vally and is a proper object to study on.At present,researcheres have begun to investigate braided rivers in the lower reaches of YJ River.However,there is still no systematic research on the geomorphology of braided river in YJ River.This paper presents an investigation on the grain size distribution and geomorphological charateristics of braided river in the wide valley of YJ River based on field observation and remote sensing technique.Sediments were collected along the river bank and on a typical central bar.The BP model of artificial neural network was used to classify the Landsat-8 and Sentinel II images.River morphological parameters such as river width,braiding intensity indices,as well as the bar number and bar size are statistically analyzed.The temporal and spatial variation are studied.Significance of main channel is analyzed in terms of channel width distribution.Then the geomorphological evolution of the braided river in YJ River is discussed.The results show that:?1?The wide vally in YJ River can be devided into three sections which are the transitional river section?R1?,the fully developed river section?R2?and the converging river section?R3?along the bank.Channel count index()of R2 can reach up to 12 and its river width can reach 3.6 km;?2?Sediments in the wide valley are mostly fine and medium sands.The grain size in most sampling spots ranges between 0.062-0.5 mm,and the median diameter is about 0.23 mm.Sediment is well sorted with single-peak grading curve.Grain size along the bank and depth does not change much;?3?In R2 section,the braided river geomophology has obvious seasonal charateristics.River width,bar number,bar size and bar braiding index vary with discharge with high synchronicity,whilelags behind the discharge and reaches its maximum at the end of flood season.;?4?Seasonal difference in the distribution of the channel width exists in R2 section.In dry season,the dominance of the widest channel is not obvious,and the development of each channel is relatively balanced.While during flood season,the widest channel in the river widens greatly and become the dominant main channel.