Biogeomorphology and diversity in multiple-channel river systems

This paper examines the causes of the high biodiversity of wooded alluvial floodplains. Multiple-channel floodplain forests have high ecotone/area ratios as a result of long stream lengths of channel banks and the presence of gravel/sand bars and palaeochannels. There is evidence that the multiple-channel state is a natural state for northwest European floodplains prior to deforestation and channelization. One of the last semi-natural forested floodplains in Europe, the Gearagh in southwest Ireland, illustrates the processes which maximize floodplain biodiversity. Of particular importance are structural factors including windthrow which increases microtopographic variation and adds irregularity to river morphology. The complexity of the system allows the co-existence of organisms with rather different ecological requirements, an example being invertebrates characteristic of both lowland and upland rivers. The system is characterized by interactions between biological and geomorphological processes with channel pattern and functioning being partially controlled by ecological processes which further increase the diversity of this relatively stable homeostatic system. It is argued that through the re-creation of multiple channels, floodplain diversity can be appreciably increased and maintained. This approach is compatible with the use of floodplains for hood storage and could therefore be a component of flood relief schemes.