Post-European changes to the fluvial geomorphology of Bega catchment, Australia: implications for river ecology

1. Within a few decades of European disturbance in the mid-nineteenth centu ry, river character and behaviour were transformed in Bega catchment on the south coast of New South Wales, Australia. Ecological impacts of geomorphi c changes to river structure and function throughout the catchment are asse ssed. 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 co ntinuous in the middle and upper parts of the catchment, as extensive valle y 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 headw ater streams, but have been dramatic along virtually all river courses beyo nd the base of the escarpment. 4. Changes in river structure have been directly related to altered riparia n vegetation cover, and vice versa. As a consequence of changes to river st ructure, bed substrate calibre (and supply volume/rate) has been modified a long 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 st ructure suggest that ecologists need to adopt a longer-term, catchment-fram ed view of human disturbance to river ecosystems. 7. Effective, sustainable ecological rehabilitation of river courses is dep endent on an understanding of geomorphic processes and determination of app ropriate river structure at differing positions in catchments.