McGregor (1986), for example, reports that in the upper Yallahs basin there is extensive sheetwash erosion, slope failures and extensive gullying.

This included a diagram of drainage basin transformation from low to high density which related the severe gullying that could occur on slopes to the aggradation along the main valley axis.

Gullying or gully erosion is the process by which gullies are formed.

Side-sloped trails are prone to gullying.

Hillsides are more prone to gullying when they are cleared of vegetation, through deforestation, over-grazing or other means.

This may have resulted from a number of factors: progressive stripping of the upper soil horizons, gullying or the introduction of plough agriculture.

Similar to drop structures in purpose, they reduce erosion and gullying in the channel and allow sediments and pollutants to settle.

The sediment geochemistry, which will be discussed in full elsewhere, implies that erosion of subsoil (fraction c in Fig. 2), possibly by gullying, was more important than the removal of fine weathered topsoil (fraction a).