Variation Process Of Water And Sediment In The Yellow River And Evolution Of Sedimentary Environment In The Yellow River Delta

The Yellow River is the second river of China, with world-famous of its high sediment-laden water. In the past, to harness the Yellow River, the first issue should be managing the river sediment, especially the river sediment from the middle reaches should be decreased as much as possible. In history, the sediment-laden water of the Yellow River has resulted in serious siltation in the lower reaches and the floodplain, which bringed heavy tragedy to people's life. However, the sediment-laden water has developed the large Yellow River Delta, which can provide available land resource for the development of society and economy. By means of statistics, wavelet analysis, regression analysis and softwares of Surfer and Mapinfo, this paper analyzes the variation processes of water discharge and sediment load in the Yellow River from 1950 to 2009, expatiates the effect of variation of water discharge and sediment load to the lower reaches and the river delta, and discusses the evolution of sedimentary environment of the Yellow River Delta. The results are shown as follows:Water and sediment in the Yellow River Basin yield from the upper-middle reaches, but have different sources. The water discharge is derived mainly from the upper reaches and the sediment load is derived mainly form the middle reaches. The gradual decreases of water discharge and sediment load in hydrological stations of the Yellow River Basin from 1950 to 2009 are strongly influenced by climatic change and human activities. The main factor to river sediment decrease is water-soil conservation practices in the middle reaches, the second contribution is attributed to sediment trapping by reservoirs, and the following contribution is precipitation decrease. Water discharge and sediment load transport into the sea by the Yellow River have three distinct time-scale period variations of annual vatiation (0.5-1.0a), inter-annual variation (3.0-6.5a) and inter-decadal variation (10.1-14.2a), and the period variation of sediment load is mainly controlled by the period variation of water discharge. Since 1970s, the three distinct time-scale period variations are unconspicuous, and with the shorter of time-scale, the lower conspicuousness of period variation. Since 1950, the lower Yellow River has experienced alternate phases of siltation and scouring due to changes of water discharge and sediment load at Huayuankou hydrological station. When the suspended sediment concentration at Huayuankou is less than 18.6kg/m3, the lower reaches tend to experience scouring. Otherwise siltation is the dominant process. Except controlled by water discharge and sediment load, siltation and scouring downwards from Aishan reaches have been affected by tranfers of river course to the sea. The reaches has experienced upriver scouring through forming new river mouth in early days of river course transfer and it has exhibited upriver siltation as extension of river mouth in mid-late time of transfer. Over most of the data period, when the ratio of total water discharge and sediment load into the sea (SSCT) locates in 25.34-26.05kg/m3 in different periods, the coastline near river mouth has extended and the delta area has increased through progradation. However, since the operation of Xiaolangdi reservoir in 1999, the SSCT as recorded at Lijin from 2000 to 2007 is only 10.90kg/m3. The value clearly indicates the estuarine delta has been starved of sediment. The coasts near abandoned river mouths have been eroding since the river sediment supply was cut off. Hence, the whole Yellow River delta has displayed erosion from siltation.The sediment types of Core YDZ1 in the Yellow River delta are sand, silty sand, sandy silt, silt and clayey silt. The ferrimagnetic mineral of pseudo-single domain (PSD) and multi-domain (MD) in size dominates the magnetic properties of the samples from Core YDZ1. According to variation characteristics of granularity parameters and magnetics parameters of sediment from Core YDZ1, and combining with dating by AMS14C, we deduce that the sedimentary facies from top to bottom in the Yellow River Delta has experienced floodplain facies, river facies, delta-front facies, neritic facies, tidal flat facies and river facies in sequence, and the sedimentary dynamical environment had exhibited variation process of relative strong (terrestrial facies), relative weak (marine facies) and relative strong (terrestrial facies).