Preprints
https://doi.org/10.5194/egusphere-2023-2767
https://doi.org/10.5194/egusphere-2023-2767
13 Dec 2023
 | 13 Dec 2023

Conceptualising surface water-groundwater exchange in braided river systems

Scott Wilson, Jo Hoyle, Richard Measures, Antoine Di Ciacca, Leanne K. Morgan, Eddie W. Banks, Linda Robb, and Thomas Wöhling

Abstract. Braided rivers can provide substantial recharge to regional aquifers, with flow exchange between surface water and groundwater occurring at a range of spatial and temporal scales. However, the difficulty of measuring and modelling these complex and dynamic river systems has hampered process understanding and the upscaling necessary to quantify these fluxes. This is due to an incomplete understanding of the hydrogeological structures which control river-groundwater exchange. In this paper, we present a new conceptualisation of subsurface processes in braided rivers based on observations of the main losing reaches of three braided rivers in New Zealand.

The conceptual model is based on a range of data including: lidar, bathymetry, coring, particle size distribution, groundwater, temperature monitoring, radon-222, electrical resistivity tomography, and fibre optic cables. The combined results indicate that sediments within the recently active river braidplain are distinctive, with sediments that are poorly consolidated and better sorted compared to adjacent deposits from the historical braidplain, which become successively consolidated and intermixed with flood silt deposits due to overbank flow. A distinct sedimentary unconformity, combined with the presence of geomorphologically distinct lateral boundaries, suggests that a “braidplain aquifer” forms within the active river braidplain through the process of sediment mobilisation during flood events.

This braidplain aquifer concept introduces a shallow storage reservoir to the river system, which is distinct from the regional aquifer system, and mediates the exchange of flow between individual river channels and the regional aquifer. The implication of the new concept is that surface water-groundwater exchange occurs at two spatial scales. The first is hyporheic and parafluvial exchange between the river and braidplain aquifer. The second is exchange between the braidplain aquifer and regional aquifer system. Exchange at both scales is influenced by the state of hydraulic connection between the respective water bodies. This conceptualisation acknowledges braided rivers as whole “river systems”, consisting of channels, and gravel aquifer.

This work has important implications for understanding how changes in river management (e.g., surface water extraction, bank modification and gravel extraction) and morphology may impact groundwater recharge, and potentially river flow, temperature attenuation, and ecological resilience during dry conditions.

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this preprint. The responsibility to include appropriate place names lies with the authors.
Scott Wilson, Jo Hoyle, Richard Measures, Antoine Di Ciacca, Leanne K. Morgan, Eddie W. Banks, Linda Robb, and Thomas Wöhling

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2023-2767', Anonymous Referee #1, 16 Feb 2024
    • AC2: 'Reply on RC1', Scott Wilson, 04 Apr 2024
  • RC2: 'Comment on egusphere-2023-2767', Anonymous Referee #2, 13 Mar 2024
    • AC1: 'Reply on RC2', Scott Wilson, 04 Apr 2024
    • AC3: 'Reply on RC2', Scott Wilson, 05 Apr 2024

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2023-2767', Anonymous Referee #1, 16 Feb 2024
    • AC2: 'Reply on RC1', Scott Wilson, 04 Apr 2024
  • RC2: 'Comment on egusphere-2023-2767', Anonymous Referee #2, 13 Mar 2024
    • AC1: 'Reply on RC2', Scott Wilson, 04 Apr 2024
    • AC3: 'Reply on RC2', Scott Wilson, 05 Apr 2024
Scott Wilson, Jo Hoyle, Richard Measures, Antoine Di Ciacca, Leanne K. Morgan, Eddie W. Banks, Linda Robb, and Thomas Wöhling
Scott Wilson, Jo Hoyle, Richard Measures, Antoine Di Ciacca, Leanne K. Morgan, Eddie W. Banks, Linda Robb, and Thomas Wöhling

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Short summary
Braided rivers are complex and dynamic systems that are difficult to understand. This paper proposes a new model of how braided rivers work in the subsurface based on field observations in 3 braided rivers in New Zealand. We suggest that braided rivers create their own shallow aquifers by moving bed sediments during flood flows. This new conceptualisation considers braided rivers as whole “river systems” with both channels and a gravel aquifer, which is distinct from the regional aquifer.