Morphological adjustments of alternate bars in a channelized river of the Tibetan Plateau and their hydro-sedimentary implications

Abstract

Alternate bars are self-organized, large-scale bedforms that typically occur in channelized rivers and result from bed instability. Owing to the rapid morphological responses to changing flow and sediment regime, alternate bars can serve as sensitive indicators for reach-scale fluvial variations. The aspect is particularly relevant for the Tibetan Plateau rivers, where climatic and human disturbances are triggering rapid fluvial adjustments. We analysed alternate bars in the lower Nianchu River – a channelized tributary in the middle Yarlung Tsangpo River basin – by utilizing multi-temporal satellite imagery (2009–2012 and 2019–2021), field surveys and theoretical modelling. Key findings include: (1) a 1.5-fold decline in bar occurrence frequency (0.99 to 0.66 bars/km) and a 2.3-fold reduction in migration rates (0.36 to 0.16 channel widths/year) are observed in the recent decade, mirroring the decreased flow energy and sediment supply due to dam regulation, vegetation encroachment and altered hydrology in the midstream rivers in the Tibetan Plateau; (2) persistent bar wavelengths despite the above bar changes, indicating that hydro-sedimentary changes are not yet fundamental to alter the basic bar pattern, providing critical insights that water-sediment adjustments of midstream rivers in the Tibetan Plateau are gradual rather than abrupt; and (3) agreement of semi-empirical wavelength equation to the field data under a scaling factor of 0.7, confirming the transferability of experimental/theoretical models for natural rivers of high-altitude in bar management and regulations. Overall, morphological changes of alternate bars can provide critical insights to understand river responses to climate change and human activities.