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

Antony G Brown,Edward J Rhodes,Steve Davis,Yusheng Zhang,Ben Pears,Nicki J Whitehouse,Chris Bradley,Jenny Bennett,Jean-Luc Schwenninger,Antony Firth,Emma Firth,Paul Hughes,Des Walling

doi:10.1016/j.quascirev.2021.106929

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.

Highlights

F1-F4, are the original channel survey maps overlaid onto the relevant sections of the GIS-based mapping of the historic watercourse and feature polygons and land use produced by Fjordr Ltd. for Historic England, reproduced here by permission, and full sources are given in the captions
The planforms of the active main channels display low sinuosity (1.1e1.5) but taking the entire active channel system this almost doubles (1.6e2.6). This is still lower than the palaeochannel sinuosity it is unlikely that the whole system was ever active simultaneously, except in floods (Table 1)
These terraces are dissected by channels and bounded by intersecting channels, effectively forming islands in the floodplain

Summary

Introduction

To Quaternary geologists rivers are geological agents directionally forced by climate and tectonics (Rittenour et al, 2007; Macklin et al, 2015; Prins and Andresen, 2019); whereas fluvial geomorphologists have been rather more focussed on the degree to which rivers can be regarded as equilibrium forms balancing discharge, sediment and slope (Leopold et al, 1964; Nanson and Huang, 2018; Chartrand et al, 2019), whilst accepting that most rivers are not in a state of equilibrium, but are self-adjusting and are subject to a range of inherited conditions (Tooth and Nanson, 2000; Brookes and Brierley, 2000; Lewin, 2011; Fryirs et al, 2016; Gallagher et al, 2018). Non-equilibrium conditions have generally been seen as arising due to forcing factors or anomalies, associated with bedrock outcrops, or large organic debris and related turbulence fluctuations (Thompson and Wohl, 2009). This paper takes a holistic approach to one lowland multiple-channel floodplain system, the Culm in SW England, over the current interglacial and assesses how its evolution has influenced its current geomorphological state, equilibrium vs non-equilibrium conditions, and ecological resilience in the face of climate and land use change

Methods

Results

Discussion

Conclusion