The negative relief of large river floodplains

Large floodplains have multiple and complex negative relief assemblages in which depressions fall below local or general floodplain surfaces at a variety of scales. The generation and dynamics of negative relief along major alluvial corridors are described and compared. Such depressions are significant for the storage and passage of surface waters, the creation of a range of riparian, wetland, lacustrine and flowing-water habitats, and the long-term accumulation of organic materials. Working on trunk channel remnants, drowned valleys and subsidence basins, fluvial processes modify floodplain negative relief through differential erosion and sedimentation. Effectively this takes place in three genetic zones: rheic, transitional and perirheic. We show that transitional zones marginal to active channels significantly diversify form complexes, and we demonstrate the diachronous nature of zonal processes and the complex nature and pace of depression modification and infilling. Four less well-understood sets of coupled phenomena are assessed: (i) floodplains associated with discontinuous river banks, (ii) the scales and types of scroll bar generation, (iii) factors underlying the contrasts between meander and braidplain surface relief, and (iv) the generation and function of large floodplain wetlands and lakes. The survival likelihood of surface negative relief relates to geomorphological connectivity; this is described for each of the rheic, transitional and perirheic zones. Implications for contemporary aquatic system management are discussed. A key to understanding and managing negative relief on large river floodplains, and their associated ecologies and sedimentation, is to quantify both sedimentological and hydrological river-floodplain connectivity.