The study focused on the question of how the natural and human forcing has affected the morphodynamic evolution of bars in the middle Yangtze River. A large channel bar was investigated using satellite remote sensing data during 1986–2017. A novel algorithm was proposed to select images acquired at the similar river stage. The bar was delineated from selected images and surface area, volume and centroid were quantified to examine bar morphodynamics. The results indicated a four-stage evolution process, including a steady extension (1986–1998), a quick shrinkage (1998–2002), a fluctuated shrinkage (2002–2012), and a fluctuated extension (2012–2017). The first stage was characterized by the steady growth at a rate of 0.087 km2/a, and a centroid translation towards the inner bend by 400 m. The second stage was the most dynamic episode, with the bar degradation at a rate of 0.71 km2/a, and a downstream migration by 850 m. In the third stage, the bar was eroded at a rate of 0.05 km2/a, and migrated towards the inner bend by about 80 m. In the fourth stage, the bar was deposited at a rate of 0.12 km2/a, and traveled upstream by around 400 m. The bar volume exhibited the similar trend. The analysis showed that the four stages were separated by the consecutive floods, the closure of Three Gorges Dam and the installation of two river engineering works. The results suggested that the bar evolved much more dynamic under the natural flow (1985–2003) than that of the regulated (2003–2012). The findings revealed that the evolution was largely governed by the flow and sediment supply while the human forcing can affect the process by altering these regimes. This study also demonstrated the great usefulness of combining multi-temporal imagery and river stage records in the assessment of bar morphodynamics.

Three decadal morphodynamic evolution of a large channel bar in the middle Yangtze River: Influence of natural and anthropogenic interferences

Intensive human activities in river basins have substantial effects on fluvial hydrological and morphological processes and developmental trends. In this study, we assessed the evolutionary characteristics and developmental trends of the fluvial morphological system of the Three Gorges Dam (TGD) downstream to understand the impacts of human activities on the fluvial hydrological–morphological system of the region. The results indicated that the Three Gorges Project (TGP) and other upstream cascade reservoirs promoted sediment retention, which in turn aggravated the trend of declining sediment load and cumulative scouring over a long distance in the channels downstream of the TGD. Approximately 84% of scouring occurred in the low‐water channels, along with thalweg scouring and riverbank collapse. Notably, extended operation of the TGP and other upstream cascade reservoirs has resulted in gradual changes in the downstream riverbeds. Specifically, we found that the most intensive scouring zone shifted downstream from the near‐dam Yichang–Chenglingji reach (0–408 km) to the Chenglingji–Hankou reach. The scouring–silting distribution in swales has also changed: the scouring of deep swales and silting on floodplains prior to the TGP have been replaced by the scouring of both deep swales and floodplains. The scouring intensity of riverbeds in the Yichang–Hukou reach (954 km) was slightly higher than that forecasted prior to the TGP; the scouring intensities increased significantly during 2013–2021, which could be attributed to the TGP and other upstream cascade reservoirs; river sand mining and waterway dredging are secondary factors. We conclude that, owing to the effects of sediment retention by cascade reservoirs in the upper reaches of the Yangtze River (or Changjiang River), the cumulative scouring intensities of riverbeds in the middle and lower reaches of the Yangtze River will continue to increase in future decades.

Impact of upstream reservoirs on geomorphic evolution in the middle and lower reaches of the Yangtze River

Channel morphologic processes of a highly sinuous bend approaching neck cutoff by bank erosion in the middle Yangtze River

River and floodplains are process-response systems that geomorphic states are typically results of hydrologic drivers and geologic setting. Large sand-bed braided rivers geomorphology shows more sensitive dynamic evolution and significant space–time difference. However, few investigations have been focused on it. Here we show that river characteristics including expansion–contraction and elevation heterogeneity of the river cross sections affect erosion and deposition partially and cause spatially differential variation of morphology in the Yanni Wetland, in the middle of Yarlung Zangbo–Brahmaputra River (YBR), and interannual hydrology process is main responsible for temporal river morphology variation as the different impact of each HIs on river morphology. We found the river morphology varied significantly with main channel shifting and branches removing over the past 35 years. Within HIs, bankfull flow (Qbf) is the channel forming discharge equal to 5.6 year flood approximately. The sub-bankfull flow (Qsf), and main channel full flow (Qmf) are channel maintaining discharge. Affected by the broad width, main channel straighten was a result of years of large floods over Qbf. Small and moderate floods within the Qsf and Qbf are also important as the morphology adjustments were mainly observed in sub-channels. And effective discharges with the Nash approach (Q1/2) are more suitable in braided river compared with Wolman-Miller approach (Qef). Our research highlighted complexity response of landform and hydrology to river morphological variations in sand-bed braided riverine wetland. It provides reference basis for studying the ecological importance of flood dispatches and complements the scarcity of research data on morphological evolution in Tibet. And a new method for assessing the HIs with remote sensing was established that is convenient with high accuracy and suitable for wide-valley river with multiple channels. We anticipate our method to be a new view to analyze ecological hydrology, and promote better managing river ecosystems. 

Effects of hydrology and river characteristics on riverine wetland morphology variation in the middle reaches of the Yarlung Zangbo–Brahmaputra river based on remote sensing

Given the importance of waterway depths in river development, the effects of the evolution of bars and troughs on waterway expansion play an important role in river management and water depth conservation. This study aims to expand the waterway dimensions of the Jingjiang Reach of the Yangtze River. To achieve this objective, determining the relationship between river evolution processes and the potential for waterway depth improvement and navigation hindrances is vital. Therefore, the sedimentation, hydrological, and terrain data of the Jingjiang Reach from 1955 to 2020 are analysed to elucidate the above-mentioned relationship. Since the commissioning of the Three Gorges Dam, the scouring of the low-flow channel has accounted for 90%–95% of all scouring in the Jingjiang Reach. Furthermore, the central bars and beaches have shrunk by 9.4% and 24.9%, respectively, and 18.3% overall. Considering the bed scouring and waterway regulation projects in the Jingjiang Reach, we investigated the continuity of a 4.5 m × 200 m × 1050 m (depth × width × bend radius) waterway along the Jingjiang Reach, and find that navigation-hindering channels account for over 5.3% of the waterway length. Furthermore, part of the Jingjiang Reach is an important nature reserve and shelters numerous water-related facilities, which inhibits the implementation of waterway deepening projects. The findings of this study demonstrate that numerous challenges are associated with increasing the waterway depths of the Jingjiang Reach. 

http://www.geogsci.com/CN/article/downloadArticleFile.do?attachType=PDF&id=53560

Relationship between potential waterway depth improvement and evolution of the Jingjiang Reach of the Yangtze River in China

Meandering-anabranching river channel change in response to flow-sediment regulation: Data analysis and model validation

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Meandering-anabranching river channel change in response to flow-sediment regulation: Data analysis and model validation