Stream morphology is an important indicator for revealing the geomorphological features and evolution of the Yangtze River. Existing studies on the morphology of the Yangtze River focus on planar features. However, the vertical features are also important. Vertical features mainly control the flow ability and erosion intensity. Furthermore, traditional studies often focus on a few stream profiles in the Yangtze River. However, stream profiles are linked together by runoff nodes, thus affecting the geomorphological evolution of the Yangtze River naturally. In this study, a clustering method of stream profiles in the Yangtze River is proposed by plotting all profiles together. Then, a stream evolution index is used to investigate the geomorphological features of the stream profile clusters to reveal the evolution of the Yangtze River. Based on the stream profile clusters, the erosion base of the Yangtze River generally changes from steep to gentle from the upper reaches to the lower reaches, and the evolution degree of the stream changes from low to high. The asymmetric distribution of knickpoints in the Hanshui River Basin supports the view that the boundary of the eastward growth of the Tibetan Plateau has reached the vicinity of the Daba Mountains.
In China's Loess Plateau severe gully erosion (LPGE) region, the shoulder-line is the most intuitive and unique manifestation of the loess landform, which divides a landform into positive and negative terrains (PNTs*The spatial combination model of PNTs is of great significance for revealing the evolution of the loess landform. This study modeled and proposed the Surface Nibble Degree (SND), which is a new index that reflects the comparison of the areas of PNTs. Based on 5 m DEMs and matched high-resolution remote sensing images, the PNTs of 172 complete watersheds in the LPGE were extracted accurately, and the SND index was calculated. The spatial distribution trend of SND was discussed, and the relationship between SND and the factors that affect the evolution mechanism of regional landform was explored further. Results show that: (1) The SND can be calculated formally. It can quantify the development of the loess landform well*2) The SND of the LPGE has evident spatial differentiation that increases from southwest to northeast. High values appear in Shenmu of Shaanxi, Shilou of Shanxi, and northern Yanhe River, whereas the low values are mainly distributed in the southern loess tableland and the inclined elongated ridge area of Pingliang in Gansu and Guyuan in Ningxia*3) In the Wuding River and Yanhe River, the SND decreases with the increase in flow length (FL*In the North-Luohe River and Jinghe River, the SND increases with FL*4) SND is significantly correlated with gully density and sediment modulus and moderately correlated with hypsometric integral. As for the mechanism factors analysis, the relationship between loess thickness and SND is not obvious, but SND increased first and then decreased with the increase of precipitation and vegetation in each geographical division, and we found that the land use type of low coverage grassland has greater erosion potential.
Lake water level is an essential indicator of environmental changes caused by natural and human factors. The water level of Poyang Lake, the largest freshwater lake in China, has exhibited a dramatic variation for the past few years, especially after the completion of the Three Gorges Dam (TGD*However, there is a lack of more accurate assessment of the effect of the TGD on the Poyang Lake water level (PLWL) at finer temporal scales (e.g., the daily scale*Here, we used three machine learning models, namely, an Artificial Neural Network (ANN), a Nonlinear Autoregressive model with eXogenous input (NARX), and a Gated Recurrent Unit (GRU), to simulate the daily lake level during 2003-2016. We found that machine learning models with historical memory (i.e., the GRU model) are more suitable for simulating the PLWL under the influence of the TGD. The GRU-based results show that the lake level is significantly affected by the TGD regulation in the different operation stages and in different periods. Although the TGD has had a slight but not very significant impact on the yearly decline of the PLWL, the blocking or releasing of water at the TGD at certain moments has caused large changes in the lake level. This machine-learning-based study sheds light on the interactions between Poyang Lake and the Yangtze River regulated by the TGD.
It is of necessity to investigate the adjustment of flood discharge capacity in the Lower Yellow River (LYR) because of its profound importance in sediment transport and flood control decision-making, and additionally its magnitude is influenced by the channel and upstream boundary conditions, which have significantly varied with the ongoing implementation of soil and water conservation measures in the Loess Plateau and the operation of the Xiaolangdi Reservoir. The braided reach between two hydrometric stations of Huayuankou and Gaocun in the LYR was selected as the study area. Different parameters in the study reach during the period 1986-2015 were calculated, covering bankfull discharge (the indicator of flood discharge capacity), the pre-flood geomorphic coefficient (the indicator of channel boundary condition), and the previous five-year average fluvial erosion intensity during flood seasons (the indicator of incoming flow and sediment regime*Functional linkages at scales of section and reach were then developed respectively to quantitatively demonstrate the integrated effects of channel and upstream boundary conditions on the flood discharge capacity. Results show that: (1) the reach-scale bankfull discharge in the pre-dam stage (1986-1999) decreased rapidly by 50%, accompanied with severe channel aggradation and main-channel shrinkage. It recovered gradually as the geometry of main channel became narrower and deeper in the post-dam stage, with the geomorphic coefficient continuously reducing to less than 15 m-1/2*2) The response of bankfull discharge to the channel and upstream boundary conditions varied at scales of section and reach, and consequently the determination coefficients differed for the comprehensive equations, with a smallest value at the Jiahetan station and a highest value (0.91) at reach scale. Generally, the verified results calculated using the comprehensive equations agreed well with the corresponding measured values in 2014-2015*3) The effect of channel boundary condition was more prominent than that of upstream boundary condition on the adjustment of bankfull discharge at the Jiahetan station and the braided reach, which was proved by a larger improvement in determination coefficients for the comprehensive equations and a better performance of geomorphic coefficient on the increase of bankfull discharge.
Tidal creeks are the main channels of land-sea ecosystem interactions, and their high dynamics are an important factor affecting the hydrological connectivity of tidal flats. Taking the Yellow River Delta as the research area, we selected remote sensing images obtained during five periods from 1998 to 2018 as the data sources. Based on the spatial analysis function in GIS, the typical morphological characteristics of tidal creeks, such as the level, length, density, curvature, bifurcation ratio, and overmarsh path length (OPL), were extracted to characterize the degree of development of the tidal creeks in the Yellow River Delta wetlands. The spatio-temporal evolution of the tidal creeks was studied, and the development process and the characteristics of the tidal creeks during the different stages of development were investigated. The results revealed that (1) The number, density, and bifurcation ratio of tidal creeks exhibit an increasing trend, but the growth of the trend is slowing. The number of tidal creeks increased by 44.9% from the initial stage of the Yellow River diversion to the late stage of the wetland restoration, but it only increased by 26.2% from the late stage of the wetland restoration to the slow expansion of the Spartina alterniflora*2) The curvature of the tidal creeks on the landward side is greater than that on the seaward side*3) The development degree of tidal creek has spatial heterogenetiy, which is Area III > Area II > Area I*4) The drainage efficiency is significantly correlated with the tidal creak density and bifurcation ratio. Based on the analysis of the various morphological parameters and the drainage efficiency, it was found that after the rapid change in the tidal creek system in the early stage, the tidal creeks entered a state of slow change, and the development state of the tidal creeks tends to be in dynamic balance. The results of this study are expected to provide scientific support for the sustainable development and utilization of coastal tidal flats.
Giant clam shell mining (GCSM), a unique phenomenon occurring at remote coral reefs in the southern South China Sea (SCS), forms striking scars on the reef flats and damages the reef flat substrate. Through image analyses at three times (2004.02.02, 2014.02.26, and 2019.04.10) and in situ surveys at Ximen Reef, a representative site that has experienced GCSM, we quantified the GCSM-generated substrate damage and the corresponding recovery. GCSM was estimated to have occurred sometime between 2012 and 2014, causing reduction in live coral subarea and formation of micro-relief as trenches and mounds. GCSM-generated damage was restricted to the reef flat. After GCSM, coral and algae subarea increased, and the trenches and mounds tended to be filled and eroded, representing a natural recovery of the substrate. The legal prohibition on human disturbances at the coral reefs contributed to substrate recovery at Ximen Reef. This case also implied that recovery of the other coral reefs that suffered from GCSM is possible.
Heavy metal pollution is hazardous for the environment and human health. However, there are few studies of heavy metal pollution caused by historic metallurgical activity. The Laoniupo site in the Bahe River valley, Guanzhong Basin, China, was an important settlement of the Shang Culture (1600-1046 BCE*We studied two stratigraphic profiles at the Laoniupo site, which were used for measurements of magnetic susceptibility, heavy metal concentrations, and AMS 14C ages to provide evidence of copper smelting activity at the site during the Shang Dynasty. The Nemerow Pollution Index and Geoaccumulation Index were calculated to assess the heavy metals record (Cu, Zn, Ni, Pb, Cr, and As) in the topsoil on the loess tableland. According to the Single Pollution Index, the topsoil was slightly polluted by As and unpolluted by Cu, Zn, Ni, Pb and Cr; according to the Nemerow Composite Pollution Index the topsoil was mildly polluted; and according to the Geoaccumulation Index, the topsoil was moderately polluted by As, slightly polluted by Cu, and unpolluted by Zn, Ni, Pb and Cr. The main cause of the heavy metal pollution in the topsoil is the presence of copper slag in the cultural layers that was disturbed by modern farming activity.
Background vales (mg/kg)
On 10th Oct. and 3rd Nov. 2018, two successive landslides occurred in the Jinsha River catchment at Baige Village, Tibet Autonomous Region, China. The landslides blocked the major river and formed the barrier lake, which finally caused the huge flood disaster loss. The hillslope at Baige landslide site has been still deforming after the 2018 slidings, which is likely to fail and block the Jinsha River again in the future. Therefore the investigation of 2018 flood disaster at the Baige landslide is of a great significance to provide a classic case for flood assessment and early warning for the future disaster. The detailed survey revealed that the outstanding inundations induced bank collapse disasters upstream the Baige landslide dams, and the field investigations and hydrological simulation suggested that the downstream of the Baige landslide were seriously flooded due to the two periods of the outburst floods. On these bases, the early warning process of potential outburst floods at the Baige landslide was advised, which contains four stages: Outburst Flood Simulating Stage, Outburst Flood Forecasting Stage, Emergency Plan and Emergency Evacuation Stage. The study offers a conceptual model for the mitigation of landslides and flood disasters in the high-relief mountainous region in Tibet.
