Abstract
Channel bars are essential landforms and their evolution is crucial to aquatic and riparian biodiversity, river’s water-sediment process, and economic development. With the development of water conservation facilities and hydropower projects, numerous changes have been taken place in hydrological regimes and morphology. There have been many changes on channel bars in the middle reaches of Hanjiang River due to the combined effects of cascade reservoirs. However, little was known about such dynamics and their linkages to cascade dams across the entire downstream area. Using Landsat remote sensing images from 1986–2018 and the threshold binary Otsu extraction method, this study completed comprehensive monitoring of nine mid-channel bars (DX1–DX7, XZ1, and XZ2), and three shoal group (XZ3–XZ5) dynamics. Results showed that the mid-channel bars’ area in the reach from Danjiangkou to Xiangyang (DX) decreased over the past 33 years, with the exception of DX4, while the total area decreased by 23.19%, this channel bars’ area change was mainly influenced by backwater from the Cuijiaying Reservoir with high water level after 2010 (r = −0.93, p < 0.01). The total channel bar area from Xiangyang to Huangzhuang (XZ) decreased by 16.63% from 1986 to 2018. The total channel bar area in XZ had a strong negative correlation with runoff at Huangzhuang hydrologic station (r = −0.79, p < 0.05), which was partly attributed to upstream precipitation according to the high correlation between runoff and precipitation (R2 = 0.65). In general, the DX section was under equilibrium between scouring and deposition compared to downstream Xiangyang, the bars in DX section were mainly affected by water level, and bars in XZ section during 1986–2018 were complicated because it was upstream eroded and downstream deposited. In addition, vegetation cover, revetments, flood events, sand mining, land use, and over-exploitation may cause channel bar area dynamics. Hence, more continuous investigations are suggested to focus on effects of cascade reservoir operation on hydrological regime, as well as the changing morphology of channel bars in the middle reaches of the Hanjiang River.
Highlights
As an important component of river morphodynamics, channel bars can preserve biodiversity in river corridors and provide habitat for certain organisms [1]
We find that the change rate of Xiangyang-Huangzhuang 33 (XZ3) was less than zero and decreasing, while XZ5 shows that bars of XZ1 and XZ2 were relatively stable with average change rates of −6.09% and
The results showed that the main factor influencing mid-channel bars in Danjiangkou to Xiangyang (DX) was water level following dam construction (r = −0.93, p < 0.01)
Summary
As an important component of river morphodynamics, channel bars can preserve biodiversity in river corridors and provide habitat for certain organisms [1]. The middle reaches of the Hanjiang River have developed a certain number of river bars, which have important influence on the stability of channel, navigation channel regulation, wetland biodiversity conservation, and agricultural production. Dams and reservoirs interrupt and modify fluvial system downstream flux of sediment through watersheds, impact the flow regime, and can alter the entire hierarchy of channel variables [2,3]. Bars protect the diversity of the river ecosystem, and provide information about riverine active processes and the sediment regime for understanding fluvial processes and their controlling factors [7]. Better understanding of dams, especially the cascade reservoirs impacts on morphological adjustments of channel bars, is required in current river management
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