Quantifying the response of rivers to Holocene sea level rise in Southern England

Southern England has been undergoing subsidence since the end of the Last Glacial Maximum due to the Glacial Isostatic Adjustment of the British Isles. Rivers typically respond to subsidence by aggrading, however many rivers along the coast of Southern England incise at their outlet and exhibit river profile convexities typical of systems experiencing a drop in base level. Studies have suggested that coastal erosion could result in such river morphology. Specifically, numerical modelling studies (Leyland and Darby, 2009; Hackney et al., 2014) have shown that it was only in the Late Holocene when small basins experienced significant channel incision at their outlets.  They demonstrated that this was due to the reduction in the rates of sea level rise which enabled knickpoint recession rates to exceed cliff retreat rates.  

Observations and understanding of the geomorphic controls, including both climatic and topographic drivers, on coastal river outlets remain limited, however. For the first time, we examine the spatial distributions of river outlets in Southern England through digital topographic analysis. In regions with similar lithologies and coastal exposure, we find that channels with the smallest basins incise at their mouth while rivers with the largest basins experience aggradation. This signal aligns to model predictions, assuming that the slope of rivers at the coast decreases with increasing basin size. We further explore the role of lithology on knickpoint magnitude and retreat by testing whether more resistant lithologies slow knickpoint retreat rates. Moreover, we investigate the influence of spatial variations in coastal erosion rates on knickpoint morphology, with greater wave heights and subsidence rates documented in the South West. Finally, we examine whether realistic estimates of coastal erosion can be made by reconstructing river profiles before the initiation of subsidence.  This contribution will enhance our understanding of how rivers respond to rising base levels, which is particularly important given the projected sea level rise in Southern England.