Morphodynamics of large anabranching rivers : the case of the Madeira River, Brazil

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2017-08-07

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Guo, Xiwei

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Anabranching channels are the dominant channel pattern of large rivers worldwide. The top nine largest rivers of the world by water discharge, in particular, all develop anabranching channels. Given the limited understanding on large river morphodynamics and the mechanisms of formation and process of anabranching channels, this thesis focuses on elucidating the morphodynamic conditions of a 150-km channel segment of the Madeira River, the largest tributary of the Amazon River and the fourth or fifth largest river in the world with anabranching channels, by extensively using field measurement and remote sensing data. The studied river channels are divided into three reaches: 1) Box-shaped channels with upstream and downstream anabranching structures; 2) A pseudo-meander with a downstream anabranching structure; 3) A single-threaded straight channel with a downstream anabranching structure. The analysis of the spatial-temporal channel changes of the three reaches since 1985 demonstrates the slow process of channel lateral migration and the stability of large anabranching rivers. Besides three sites with special geomorphological settings, more than 87% of the channel analyzed did not have traceable lateral migration. The area being eroded and deposited stayed relatively equivalent until after 2010, in which channel incision (erosion) significantly exceeded the amount of deposition, possibly due to the closure of two large dams on the Madeira mainstem upstream. Channel morphology and flow scheme vary largely among different channel structures, while geologic controls play an important role in a number of places that result in channel deepening and suspension of channel migration. The three reaches are dominated by nearly upright banks, which makes the channel width stay constant with increasing discharge and impedes channel-floodplain interactions. Bed shear stress, stream power, and sediment transport are further analyzed and discussed. The hydro-geomorphological features of two anabranching structures, one in reach 2 and one in reach 3, each demonstrates floodplain avulsion (erosion-triggered) and in-channel accretion (deposition-triggered), which are the two mechanisms of anabranching channel formation.

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