Alluvial gully erosion: an example from the Mitchell fluvial megafan, Queensland, Australia

Considerable attention has been focused on the role of gullies as a contributor to contemporary sediment loads of rivers in Australia. In southern Australia rapid acceleration of hillslope gully erosion has been widely documented in the post-European period (sim last 200 years). In the northern Australian tropics, however, gully erosion processes operating along alluvial plains have not been well documented and can differ substantially from those gullies eroding into colluvium on hillslopes. Aerial reconnaissance surveys in 2004 along 13 500 km of the main stem rivers that drain into the Gulf of Carpentaria (GoC), identified extensive areas of alluvial lands that have been impacted by a pervasive form of gully erosion. More detailed remote sensing based mapping within the 31 000 km2 Mitchell River fluvial megafan has identified that active gullying into alluvium occupies sim 0紥 (129 km2) of the lower Mitchell catchment. 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 also influence the distribution of gullies. In this paper we present a conceptual model of alluvial gullies, and contend that they represent a distinct end member in the continuum of gully forms that have been described in the geomorphic literature. An understanding of the processes driving this form of alluvial gullying can only be gained when they are differentiated from widely described colluvial hillslope gully models and theories. We present evidence of type examples of alluvial gullying in the Mitchell, and through analysis of their distribution and morphology at different scales, highlight some of the key mechanisms that are potentially initiating these features and driving their expansion.Griffith Sciences, Griffith School of EnvironmentFull Tex

The hydrogeomorphic influences on alluvial gully erosion along the Mitchell River fluvial megafan

Hydrogeomorphic processes influencing alluvial gully erosion were evaluated at multiple spatial and temporal scales across the Mitchell River fluvial megafan in tropical Queensland, Australia. Longitudinal changes in floodplain inundation were quantified using river gauge data, local stage recorders and HEC-RAS modelling based on LiDAR topographic data. Intra- and interannual gully scarp retreat rates were measured using daily time-lapse photographs and annual GPS surveys. Erosion was analysed in response to different water sources and associated erosion processes across the floodplain perirheic zone, including direct rainfall, infiltration-excess runoff, soil-water seepage, river backwater and overbank flood inundation. The frequency of river flood inundation of alluvial gullies changed longitudinally according to river incision and confinement. Near the top of the megafan, flood water was contained within the macrochannel up to the 100-year recurrence interval, but river backwater still partially inundated adjacent gullies eroding into Pleistocene alluvium. In downstream Holocene floodplains, inundation of alluvial gullies occurred beyond the 2- to 5-year recurrence interval and contributed significantly to total annual erosion. However, most gully scarp retreat at all sites was driven by direct rainfall and infiltration-excess runoff, with the 24-h rainfall total being the most predictive variable. The remaining variability can be explained by seasonal vegetative conditions, complex cycles of soil wetting and drying, tension crack development, near-surface pore-water pressure, soil block undermining from spalling and overland flow, and soil property heterogeneity. Implications for grazing management impacts on soil surface and perennial grass conditions include effects on direct rainfall erosion, water infiltration, runoff volume, water concentration along tracks, and the resistance of highly dispersible soils to gully initiation or propagation under intense tropical rainfall.Griffith Sciences, Griffith School of EnvironmentNo Full Tex

Degradation of the Mitchell River fluvial megafan by alluvial gully erosion increased by post-European land use change, Queensland, Australia

Along low gradient rivers in northern Australia, there is widespread gully erosion into unconfined alluvial deposits of active and inactive floodplains. On the Mitchell River fluvial megafan in northern Queensland, river incision and fan-head trenching into Pleistocene and Holocene megafan units with sodic soils created the potential energy for a secondary cycle of erosion. In this study, rates of alluvial gully erosion into incipiently-unstable channel banks and/or pre-existing floodplain features were quantified to assess the influence of land use change following European settlement. Alluvial gully scarp retreat rates were quantified at 18 sites across the megafan using recent GPS surveys and historic air photos, demonstrating rapid increases in gully area of 1.2 to 10 times their 1949 values. Extrapolation of gully area growth trends backward in time suggested that the current widespread phase of gullying initiated between 1880 and 1950, which is post-European settlement. This is supported by young optically stimulated luminescence (OSL) dates of gully inset-floodplain deposits, LiDAR terrain analysis, historic explorer accounts of earlier gully types, and archival records of cattle numbers and land management. It is deduced that intense cattle grazing and associated disturbance concentrated in the riparian zones during the dry season promoted gully erosion in the wet season along steep banks, adjacent floodplain hollows and precursor gullies. This is a result of reduced native grass cover, increased physical disturbance of soils, and the concentration of water runoff along cattle tracks, in addition to fire regime modifications, episodic drought, and the establishment of exotic weed and grass species. Geomorphic processes operating over geologic time across the fluvial megafan predisposed the landscape to being pushed by land used change across an intrinsically close geomorphic threshold towards instability. The evolution of these alluvial gullies is discussed in terms of their initiation, development, future growth, and stabilisation, and the numerous natural and anthropogenic factors influencing their erosion.Griffith Sciences, Griffith School of EnvironmentFull Tex

Using remote sensing to quantify sediment budget components in a large tropical river - Mitchell River, Gulf of Carpentaria

The recent drought in southern Australia and the increasing realisation that this may be the norm as a result of global warming has led to increased interest in developing the land and water resource of the tropical savannah in northern Australia. Intensive land and water resource development in tropical savannahs in other parts of the world has been shown to lead to significant land degradation, and there are signs this has already occurred with the comparatively moderate levels of historic grazing and agriculture in northern Australia. It is critical that we understand contemporary and historical landscape dynamics before land use is intensified in this region. As a way of beginning to quantify key sediment sources and to start to understand potential changes in relative contributions associated with land use and other drivers, we have begun to construct a sediment budget for the Mitchell River, a 71 360 km2 catchment draining into the Gulf of Carpentaria. We present some preliminary results from two key components of the sediment budget: alluvial gully erosion and channel erosion. A remote sensing analysis has identified around 130 km2 of active alluvial gullies within the Mitchell megafan, which has an estimated active front length of around 5560 km. Preliminary measurements suggest the average rate of scarp retreat to be 0.34 m per year, with scarp heights ranging between 0.3 and 8 m. Using a spatially distributed method we calculated an annual catchment sediment yield from alluvial gullying of >5 Mt/year. Estimates of minimum river channel bed turnover were also determined using a remote sensing approach from Landsat archival data. A measure of minimum annual bed turnover of 15 Mm3/year was derived from analysis of bed area change between 1988 and 2005 (i.e. from either pools to sand bars and vice versa, or vegetated island to pools and vice versa). When analysed at a tributary scale, these data show a pattern of net sediment scour from the Walsh and Palmer rivers, and net accumulation within all other mainstem channels in the period between 1988 and 2005. These data support the anecdotal evidence that there has been a trend towards sediment accumulation within the lower Mitchell River in recent times, with a resulting net reduction in total pool area of 0.6 km2. While preliminary, these data highlight the value of remote sensing for constructing sediment budgets in large dynamic tropical rivers.Griffith Sciences, Griffith School of EnvironmentNo Full Tex