HOLOCENE FLOODPLAIN METAMORPHOSIS IN THE MIDLANDS, UNITED-KINGDOM

Many lowland rivers in the United Kingdom, including the Nene, Soar and Severn, have layered floodplains with a basal gravel of Pleistocene or Late glacial age and a structureless silty clay superficial unit burying the entire former floodplain. This burial is illustrated by the existence of variable, mixed and pedologically disturbed sediments and palaeo landsurfaces between the basal gravels and superficial silty clay. This paper presents a comparison of the pre- and post-late Holocene palaeo landsurfaces and palaeochannels using data from the Nene, Soar and Severn valleys. From this comparison it is argued that during the mid to late Holocene (ca 4500 yr BP to 2500 yr BP) floodplains and river channels underwent a metamorphosis. This is indicated by accelerated vertical accretion, a reduction in floodplain relative relief, changed floodplain soil conditions, a reduction in channel W/D ratios and a resultant increase in the silty clay proportion of channel perimeter sediments. There are indications that hydrological change preceded this metamorphosis but the primary cause was an increase in fine sediment supply during the later Holocene and a disequilibrium between channel bed and floodplain aggradation rates resulting in relative incision. This metamorphosis, which is explained in this paper by the proposed stable-bed aggrading-banks model (SBAB), is the key factor in the Holocene evolution of low-energy floodplain systems in the United Kingdom, upon which more subtle short-term fluctuations are superimposed.