|Title||Alluvial gully erosion rates across the Mitchell River fluvial megafan, Queensland, Australia|
|Publication Type||Conference Participation|
|Year of Publication||2009|
|Authors||Shellberg, J, Brooks, A, Spencer, J, Knight, J|
|Keywords||4: Material Budgets|
Detailed remote sensing mapping within the 31,000 km2 Mitchell River fluvial megafan has identified that active gullying into alluvium occupies ~ 0.4% (129 km2) of the lower Mitchell catchment, with an estimated active front length of around 5560 km. Gully erosion forms and processes operating along these tropical alluvial plains can differ substantially from those gullies eroding into colluvium on hillslopes. These alluvial gullies are concentrated along main drainage channels and their scarp heights are highly correlated to the local relief between the floodplain and river thalweg. While river incision into the megafan since the Pleistocene has developed the relief potential for erosion, other factors such as floodplain hydrology, soil dispersibility, and vegetation resistance also influence the distribution of gullies and erosion processes. This talk will emphasize both recent GPS measurements (2005 to 2009) and historic air photo analysis (1943 to 2006) of alluvial gully retreat rates at head scarps. Recent GPS measurements estimated the average annual rate of scarp retreat to be 0.34 m per year across 25,485m of gully front. Maximum rates exceeded 15m per year, with scarp heights ranging between 0.3 and 8 m. Annual erosion rates calculated from historic photos were comparable to this average within the same order of magnitude. Using these rates backward in time (pre-1949), the point and time of origin were calculated for a dozen gully sites. These results indicate that many gullies initiated near or after European settlement, suggesting the contribution of land use intensification (cattle grazing and roads) to either gully initiation or acceleration. A conceptual model of these alluvial gullies has been developed, that in combination with these rate data, demonstrates that 1) alluvial gullies are a distinct end member in the continuum of gully forms described in the geomorphic literature, and 2) are sensitive to land use change.