Gullies in the landscape

In this short review we concentrate on the initiation (and therefore causation) of gullying and the function of gullies in the landscape. Gullies appear to be ‘non-natural’ features associated with disturbance of natural systems. In temperate areas significant disturbance of natural vegetation is necessary to generate sufficient shear stress to cause initiation. The work of Prosser emphasises the role that overgrazing can play in this process. In more sensitive semi-arid environments, incision is likely the result of overgrazing or periods of drought damaging valley-bottom vegetation. In the Karoo, South Africa, simulated rainfall experiments show bare ground to generate 10x more runoff compared to vegetated areas. The direct agency of humans may also be responsible for incision e.g. wagon tracks in South Africa and centuries of vehicle traffic leading to the sunken lanes of parts of southern England, Poland and Belgium. There is some evidence that gully systems develop quickly and then stabilize. Gully systems act to facilitate the movement of runoff and sediments (and in some cases, pollutants) from hillslopes to valley bottoms. Once established, significant changes to valley-bottom hydrology occur: modest rainfall events can then lead to flooding. Gullies connect areas of erosion to the valley bottoms and may lead to sharp increases in sedimentation in dams and reservoirs. Gullies are often assumed to be the source of sediments that pass along their lengths. Ephemeral gullies contribute large but varying amounts of sediment to total sediment yield. However, established gully systems may be relatively inactive in term of sediment production. Evidence from the Karoo suggests long periods of transport along, but little erosion within, gullies. Reactivation is however possible and is likely driven by changing climate