Topographic, Hydraulic, and Vegetative Controls on Bar and Island Development in Mixed Bedrock‐Alluvial, Multichanneled, Dryland Rivers

D J Milan,G L Heritage,S Tooth

doi:10.1029/2019wr026101

D J Milan, G L Heritage + Show 1 more

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https://doi.org/10.1029/2019wr026101

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Abstract

AbstractWe investigate processes of bedrock‐core bar and island development in a bedrock‐influenced anastomosed reach of the Sabie River, Kruger National Park, eastern South Africa. For sites subject to alluvial stripping during an extreme flood event (~4,470–5,630 m3 s−1) in 2012, preflood and postflood aerial photographs and LiDAR data, 2‐D 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 (~4,500 m3 s−1), most sediment (sand, fine gravel) is mobile but localized deposition on bedrock topographic highs is possible. At lower simulated discharges (<1,000 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 Kruger National Park 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 Kruger National Park rivers, but similar processes of bar/island development likely occur along numerous other bedrock‐influenced rivers across dryland southern Africa and farther afield.

Highlights

We investigate processes of bedrock‐core bar and island development in a bedrock‐influenced anastomosed reach of the Sabie River, Kruger National Park, eastern South Africa
We investigate the controls on bedrock‐core bar and island development in a bedrock‐influenced, anastomosed reach of the Sabie River (Figure 1a)
Contrary to the assumption of near‐uniform, ubiquitous sedimentation on bedrock topographic highs that underpins previous conceptual models, our data suggest that bedrock‐core bars and islands develop as a result of preferential sedimentation in local bedrock topographic lows

Summary

Introduction

Considerable recent research attention has been given to the dynamics of sediment cover in bedrock or mixed bedrock‐alluvial rivers, including in field investigations (e.g., Goode & Wohl, 2010; Hodge et al, 2011; Hodge & Hoey, 2016), laboratory flume experiments (e.g., Chatanantavet & Parker, 2008; Finnegan et al, 2007; Hodge et al, 2011; Hodge & Hoey, 2016; Johnson & Whipple, 2007), and numerical modeling studies (e.g., Huda & Small, 2014; Johnson, 2014; Sklar & Dietrich, 2004). Most researchers have assumed that over long time periods a negative feedback exists, whereby enhanced bedrock erosion is counteracted by development of a protective sediment cover (e.g., Lague, 2014; Nelson & Seminara, 2011; Sklar & Dietrich, 2004). The flume experiments of Johnson and Whipple (2007) and Finnegan et al (2007) indicate a positive feedback mechanism, whereby the locations of sediment over the bedrock are associated with increased erosion rates. In these types of rivers, the patterns, processes, and rates of bedform, barform, and island development may be crucial controls on bedrock exposure and its longer‐term susceptibility to erosion (Meshkova & Carling, 2012; Tooth & McCarthy, 2004)

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