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have also been defined as valley-bottom gullies and bank gullies. They are often as wide or
wider than they are long forming large gully complexes and “breakaway” scarps, due to the
lack of structural control on their lateral expansion (Brooks, Shellberg et al. 2009).
Gullying is a natural process, but can also be greatly accelerated by human land use factors.
Natural factors influencing gully erosion include the driving forces in erosion and sediment
transport (topography, slope, relative relief (height) and potential energy, kinetic energy from
rainfall-runoff and flood water) and resisting forces (soil texture, chemistry and erodibility,
vegetation cover) (Brooks, Shellberg et al. 2009; Shellberg, Brooks et al. 2012) . Human
impacts from land use such as grazing and changes in the fire regime can change the
vegetative resistance to surface erosion, and subsequently accelerate gully erosion (Graf
1979; Prosser and Slade 1994). Linear cattle pads (tracks) and vehicle roads can also
concentrate overland flow and increased the local shear stress needed to erode gullies
(Brooks, Shellberg et al. 2009; Shellberg, Brooks et al. 2010; Shellberg 2011).
As a local example in the lower Mitchell catchment, cattle grazing intensity and impacts are
heavily concentrated along “waterway frontages” or riparian zones of main rivers and
tributaries during the long dry season. Many alluvial gullies across the lower Mitchell initiated
post-European settlement, when traditional Indigenous management was suppressed and
cattle numbers increased significantly from the 1880s onwards (Shellberg, Brooks et al.
2010; Shellberg 2011). Gully initiation points were located at relatively steep river banks or
within un-channelled floodplain-hollows, where cattle impacts such as overgrazing, soil
disturbance, and concentration of water along cattle pads were the greatest. The long-term
evolution of the landscape created the template for gully erosion potential (relief, climate, soil
chemistry, etc.), while shorter-term changes in vegetative cover and soil erosion resistance
pushed the landscape across a stability threshold.
3.2.11.2 Gully erosion at Oriners and Sefton Stations
At Oriners and Sefton Stations, hillslope or colluvial gullies can also be found eroding into
shallow hillslopes and colluvium/alluvium mixtures across the Tertiary sediments of the
Holroyd Plain (Balurga and Mottle, Annaly land systems) (Galloway et al. 1970). The
“breakaways” referred to by Galloway et al. (1970) in the Annaly land system are actually
weathered bedrock breaks in slope or shallow escapements in Tertiary sediments, rather
than more typical alluvial “breakaways” and gullies found on Quaternary floodplains in the
Mitchell (Brooks, Shellberg et al. 2009). Besides initial field reconnaissance surveys for soil
and geological mapping (Isbell, Webb et al. 1968; Galloway, Gunn et al. 1970; Grimes and
Doutch 1978; Grimes 1979; Smart, Grimes et al. 1980a; Smart, Grimes et al. 1980a; Smart,
Grimes et al. 1980b; Biggs and Philip 1995a; Biggs and Philip 1995b), no data or detailed
observations exist for the condition of soil or gully erosion across much of the remote Holroyd
Plain away from roads and valleys, which includes most of Oriners and Sefton Stations.
More commonly at Oriners and Sefton Stations, alluvial gullies can be found along the
Quaternary floodplains, creeks and lagoon banks of Eight Mile Creek, Crosbie Creek, and
other smaller tributary valleys. Alluvial gullies are especially widespread along the mainstem
of the Alice River (Brooks, Shellberg et al. 2009). For this section, alluvial gully erosion along
creek and lagoon banks and roads near Oriners Station and Lagoon will be focused on, due
to the few available observations here and some pressing management issues.
At Oriners Lagoon (and others like Jewfish), amphitheatre-shaped alluvial gullies into lagoon
banks do exist (Figure 71ab), similar to elsewhere in the Mitchell (Brooks, Shellberg et al.
2009; Shellberg 2011). Often these gullies are created by grazing impacts like cattle pads to
access water. However in the case of Oriners, which has been occupied recently by
dispersed wild cattle in relatively low numbers, these impacts are not severe. More
commonly in the Oriners area, these bank gullies are caused by vehicle roads and tracks
(Figure 71cd) that funnel water along the floodplain and concentrate the runoff over banks
into lagoons.
Working Knowledge at Oriners Station, Cape York
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