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

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.