Discriminating between the roles of late Pleistocene palaeodischarge and geological‐topographic inheritance in fluvial longitudinal profile and channel development

Abstract

This paper investigates the influences of palaeohydrology and geological‐topographic inheritance in shaping the channel of the lower River Suir, southeast Ireland. Results of acoustic surveys of the lower River Suir and Waterford Harbour reveal two scales of pseudo‐cyclic river bedforms. Longitudinal elevation profiles of the geological topography (undulating bedrock and till‐mantled bedrock) bounding the present floodplain swath reveal pseudo‐cyclicity in that terrain too. Spectral and statistical analyses are used to quantify the cyclicity of the long profile and geological‐topographic series. These methods show that the dominant cyclicity of the long profile reflects autocorrelation more than inheritance of cyclicity from the bounding geological topography. The cyclicity of the long profile mainly reflects a hydraulic control on pool‐spacing, although some cyclicity probably has been inherited from the geological‐topography. Channel‐forming palaeodischarge is estimated based on the dominant pool‐spacing revealed by spectral analysis, validated using relationships between meander wavelength, channel cross‐sectional geometry and hydraulically‐informed discharge reconstruction. The palaeodischarge estimates are in close agreement and are two orders of magnitude greater than present flood maxima. Significantly, these palaeodischarge estimates also agree closely with palaeodischarge calculated for the submerged Pleistocene palaeochannel that extends across the near‐shore continental shelf from Waterford Harbour. The pool‐sequence of the lower Suir and the submerged palaeochannel represent a former land‐system that was active during a period of low relative sea level during the last glacial. More broadly, the paper offers insights into the landscape evolution of formerly glaciated regions that experienced very wide discharge variability during and after the transition from glacial to interglacial regimes, in a context of complex relative sea level change. Copyright © 2017 John Wiley & Sons, Ltd.

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

“…The data presented here, also suggests that in this system many of the riffles or within-channel bars and islands cannot be explained as purely equilibrium forms, but rather that the local increase in bed height caused flow divergence and gravel deposition on channel-bed steps and highs. This mechanism is not without precedence, as some pseudo-cyclic river bedforms on the river Suir in Ireland appear to have been inherited from the bounding geological-topography although this was not dominant (Gallagher et al, 2018). In fact 'forcing elements' for pool-riffle forms had already been recognised for semi-alluvial systems (alluvial fans and mixed-bed channels and bedrock outcrops), log jams, and tributary junctions (White et al, 2010).…”

“…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).…”