BIOGEOMORPHOLOGY AND DIVERSITY IN MULTIPLE-CHANNEL RIVER SYSTEMS

This paper examines the causes of the high biodiversity of wooded allu vial floodplains. Multiple-channel floodplain forests have high ecoton e/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 Euro pean floodplains prior to deforestation and channelization. One of the last semi-natural forested floodplains in Europe, the Gearagh in sout hwest Ireland, illustrates the processes which maximize floodplain bio diversity. Of particular importance are structural factors including w indthrow which increases microtopographic variation and adds irregular ity to river morphology. The complexity of the system allows the co-ex istence 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 biologica l and geomorphological processes with channel pattern and functioning being partially controlled by ecological processes which further incre ase the diversity of this relatively stable homeostatic system. It is argued that through the re-creation of multiple channels, floodplain d iversity can be appreciably increased and maintained. This approach is compatible with the use of floodplains for hood storage and could the refore be a component of flood relief schemes.