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

Milan, DJ; Tooth, S; Heritage, GL

Abstract

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.

Citation

Milan, D., Tooth, S., & Heritage, G. (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. https://doi.org/10.1029/2019wr026101

• Journal Article Type: Article
• Acceptance Date: Mar 11, 2020
• Online Publication Date: Mar 16, 2020
• Publication Date: May 14, 2020
• Deposit Date: Apr 9, 2020
• Publicly Available Date: Jun 1, 2020
• Journal: Water Resources Research
• Print ISSN: 0043-1397
• Electronic ISSN: 1944-7973
• Publisher: American Geophysical Union
• Volume: 56
• Issue: 5
• Pages: e2019WR026101
• DOI: https://doi.org/10.1029/2019wr026101
• Keywords: Water Science and Technology
• Publisher URL: https://doi.org/10.1029/2019wr026101
• Related Public URLs: http://www.agu.org/journals/wr/
• Additional Information: 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, , http://onlinelibrary.wiley.com.salford.idm.oclc.org/termsAndConditions#vor; Funders : Natural Environment Research Council (NERC);SanParks; Grant Number: NE/K001132/1