Late Quaternary evolution of a lowland anastomosing river system: geological-topographic inheritance, non-uniformity and implications for biodiversity and management

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Authors

  • AG Brown
  • EJ Rhodes
  • S Davis
  • Y Zhang
  • B Pears
  • NJ Whitehouse
  • J Bennett
  • JL Schwenninger
  • A Firth
  • P Hughes
  • D Walling

Colleges, School and Institutes

Abstract

Lowland multiple-channel rivers are characterised by floodplain-corridor heterogeneity, high ecological and heritage value, and can be in quasi-stable states. This holistic study of a surviving temperate zone example (Culm, UK) uses geomorphological mapping, 14C, direct sediment dating (OSL, fallout radionuclides), and palaeoecology. This reveals the evolution of a channel-floodplain system from an initial braided state in the Late Pleistocene to its late Holocene anastomosing state. After the Pleistocene-Holocene transition the reduced channel system incised into its braid-plain, only able to rework gravels locally due to reduced competence in relation to inherited bounding sediment calibre. This resulted in the creation of terrace islands, palaeochannels, and a stable anastomosing pattern dominated by channel junctions, bifurcations and palaeochannel intersections. Survey, coring and excavation reveal a persistence of mid-channel bars and riffles at channel junctions, and where channels crossed palaeochannel fills. In common with most other European lowland rivers this system evolves in the later Holocene due to both climate and catchment changes with a major hydrological critical transition in the mid-Holocene (c. 5300 BP). However, in the case of the Culm, the increase in fine sediment supply often seen in lowland catchments in the Middle-Late Holocene, occurred later, and was insufficient to convert the system to a single medium-low sinuosity channel-floodplain. This allowed the persistence of high heterogeneity and biodiversity (including the persistence of riffle beetles) as part of multiple-scales of non-uniformity. Indeed the pool-riffle persistence is an example of this system's non-uniformity, being due, at least in part, to the effects of previous channel history. This paper reveals why this river survived in a multichannel state, and by implication, why others did not. These results are being used in the bespoke eco-heritage management of the Culm, but could also inform the restoration of other former multi-channel lowland temperate river systems worldwide.

Bibliographic note

Funding Information: The authors acknowledge initial funding from UKRI- NERC under NER/A/2000/00394/ and later support from the Universities of Exeter and Southampton, UK. Rothamsted Research receives strategic funding from UKRI -BBSRC (UK Research and Innovation – Biotechnology and Biological Sciences Research Council , and Y. Zhang is currently funded by project BBS/E/C/000I0330 – Soil to Nutrition Project 3 . The Historic Watercourse: River Culm project carried out by Fjordr Limited is funded by Historic England through its Heritage Protection Commissions programme . Thanks go to R. Fletcher for the pollen and macrofossil analyses, A. Ames and J. Grapes for laboratory work, Z. Ruiz for comments on chironomids, L. Basell and O. Bayer for their comments on the lithics, R. Scrivener for geological expertise. Our gratitude is also extended to the many landowners and the Woodland Trust for allowing access to the study sites and L. Aspden for cartographic assistance. Thanks also to very helpful comments by G. Nanson, P. Carling, P. Bishop. El. Wohl and S. Tooth as well as the referees. This paper is dedicated to the memory of Ken Gregory CBE (1938–2020) who both worked on the Exe/Culm and commented on this project in its early stages. Publisher Copyright: © 2021 The Authors

Details

Original languageEnglish
Article number106929
Number of pages29
JournalQuaternary Science Reviews
Volume260
Early online date18 Apr 2021
Publication statusPublished - 15 May 2021

Keywords

  • Catchment change, Coleoptera, Floodplain ecology, Landform non-uniformity, Rewilding, River corridors