Topographic, hydraulic, and vegetative controls on bar and island development in mixed bedrock‐alluvial, multi‐channeled, dryland rivers

Milan, DJ ORCID:, Tooth, S and Heritage, GL ORCID: 2020, 'Topographic, hydraulic, and vegetative controls on bar and island development in mixed bedrock‐alluvial, multi‐channeled, dryland rivers' , Water Resources Research, 56 (5) , e2019WR026101.

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We investigate processes of bedrock‐core bar and island development in a bedrock‐influenced anastomosed reach of the Sabie River, Kruger National Park (KNP), eastern South Africa. For sites subject to alluvial stripping during an extreme flood event (~4470‐5630 m3 s‐1) in 2012, pre‐ and post‐flood aerial photographs and LiDAR data, 2D morphodynamic simulations, and field observations reveal that the thickest surviving alluvial deposits tend to be located over bedrock topographic lows. At a simulated peak discharge (~4500 m3 s‐1), most sediment (sand, fine gravel) is mobile but localized deposition on bedrock topographic highs is possible. At lower simulated discharges (<1000 m3 s‐1), topographic highs are not submerged, and deposition occurs in lower elevation areas, particularly in areas disconnected from the main channels during falling stage. Field observations suggest that in addition to discharge, rainwash between floods may redistribute sediments from bedrock topographic highs to lower elevation areas, and also highlight the critical role of vegetation colonization in bar stability, and in trapping of additional sediment and organics. These findings challenge the assumptions of preferential deposition on topographic highs that underpin previous analyses of KNP river dynamics, and are synthesized in a new conceptual model that demonstrates how initial bedrock topographic lows become topographic highs (bedrock core‐bars and islands) in the latter stages of sediment accumulation. The model provides particular insight into the development of mixed bedrock‐alluvial anastomosing along the KNP rivers, but similar processes of bar/island development likely occur along numerous other bedrock‐influenced rivers across dryland southern Africa and farther afield.

Item Type: Article
Additional Information: ** Article version: VoR ** From Crossref via Jisc Publications Router **Journal IDs: pissn 0043-1397; eissn 1944-7973 **History: issued 16-03-2020; published_online 16-03-2020 **License for this article: starting on 16-03-2020, ,
Schools: Schools > School of Environment and Life Sciences
Journal or Publication Title: Water Resources Research
Publisher: American Geophysical Union (AGU)
ISSN: 0043-1397
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Funders: Natural Environment Research Council (NERC), SanParks
SWORD Depositor: Publications Router
Depositing User: Publications Router
Date Deposited: 09 Apr 2020 09:44
Last Modified: 16 Feb 2022 04:20

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