Late Pleistocene-Holocene river dynamics at the Trent-Soar confluence, England, UK

Although river confluences have received geomorphic attention in recent years it is difficult to upscale these studies, so confluence-dominated reaches are commonly presumed to be either: (1) braided; or (2) meandering and characterized by laterally migrating channels. If the geomorphology of a confluence zone is to be considered over longer timescales, changes in river style need to be taken into account. This paper uses a combination of remote sensing techniques (LiDAR, GPR, ER), borehole survey and chronometric dating to test this differentiation in the confluence-zone of a medium-sized, mixed-load, temperate river system (Trent, UK), which on the basis of planform evidence appears to conform to the meandering model. However, the analysis of 'confluence sediment body stratigraphy' demonstrates that the confluence does not correspond with a simple meander migration model and chronostratigraphic data suggests it has undergone two major transformations. Firstly, from a high-energy braid-plain confluence in the Lateglacial (25-13 K yrs cal BP), to a lower-energy braided confluence in the early to middle Holocene (early Holocene-2.4 kyr BP), which created a compound terrace. Second, incision into this terrace, creating a single-channel confluence (2.4-0.5 kyr cal BP) with a high sinuosity south bank tributary (the River Soar). The confluence sediment-body stratigraphy is characterized by a basal suite of Late Pleistocene gravels bisected by younger channel fills, which grade into the intervening levee and overbank sediments. The best explanation for the confluence sediment body stratigraphy encountered is that frequent switching (soft-avulsions sensu Edmonds et al., 2011) of the tributary are responsible for the downstream movement of the channel confluence (at an average rate of approximately 0.5 m per year) dissecting and reworking older braid-plain sediments. The late Holocene evolution of the confluence can be seen as a variant of the incisional-frequent channel reorganization (avulsion) model with sequential downstream migration of the reattachment point. Copyright