OPINION Post-European changes to the fluvial geomorphology of Bega catchment, Australia: implications for river ecology GARY J. BRIERLEY, TIM COHEN, KIRSTIE FRYIRS AND ANDREW BROOKS School of Earth Sciences, Macquarie University, North Ryde, New South Wales 2109, Australia SUMMARY 1. Within a few decades of European disturbance in the mid-nineteenth century, river character and behaviour were transformed in Bega catchment on the south coast of New South Wales, Australia. Ecological impacts of geomorphic changes to river structure and function throughout the catchment are assessed. 2. At the time of European settlement, many water courses in Bega catchment were discontinuous, with extensive swamps along middle and upper courses. Following a series of direct and indirect human impacts, channels became continuous in the middle and upper parts of the catchment, as extensive valley fills at the base of the escarpment were incised. Along the lowland plain, the channel widened by over 300%, fundamentally altering the relationship between the channel and its adjacent floodplain. 3. Geomorphic changes to river structure have modified habitat availability throughout Bega catchment. The impacts have been least pronounced in headwater streams, but have been dramatic along virtually all river courses beyond the base of the escarpment. 4. Changes in river structure have been directly related to altered riparian vegetation cover, and vice versa. As a consequence of changes to river structure, bed substrate calibre (and supply volume/rate) has been modified along most streams. 5. A series of indirect, secondary impacts have modified habitat viability along river courses. Lateral, longitudinal and vertical linkages within the river system have been altered, affecting the transfer of water, sediment, organic matter, nutrients and other biotic interactions. 6. These direct and indirect consequences of geomorphic changes in river structure suggest that ecologists need to adopt a longer-term, catchment-framed view of human disturbance to river ecosystems. 7. Effective, sustainable ecological rehabilitation of river courses is dependent on an understanding of geomorphic processes and determination of appropriate river structure at differing positions in catchments. Introduction Conservation of biodiversity in aquatic ecosystems is contingent on maintenance of habitat availability along river courses. In turn, habitat availability is conditioned by river structure, which is innately determined by the geomorphic character and beha- viour of rivers (e.g. Brussock et al., 1985; Gregory et al., 1991; Fisher, 1997; Richards et al., 1997). Anthro- pogenic disturbance to riparian zones and adjacent slopes, and associated adjustments to flow and bed- load quantity or calibre, can transform the physical character and stability of a river. This can directly modify the character and pattern of riparian vegeta- tion, input of woody debris, and habitat formation. Freshwater Biology (1999) 41, 839±848 ã 1999 Blackwell Science Ltd. 839 *Correspondence and present address: Gary J. Brierley, School of Earth Sciences, Macquarie University, North Ryde, New South Wales 2109, Australia. E-mail: gbrierli@laurel.ocs.mq.edu.au