Controlling factors of the size and location of large gully systems: A regression-based exploration using reconstructed pre-erosion topography

- Gullies apparently initiated as bank gullies in relatively steep topography.
- Size is directly related to contributing area measured in gully outlets.
- Evolution occurred mainly in the form of headcut retreat and bifurcation.
- Evolution was apparently dominated from an early stage by mass movements.

The role of different factors potentially affecting the size and location of large gully systems is explored in two tributary basins of the Tagus River (S Portugal). The hillslopes of these basins, corresponding to tertiary sedimentary formations, are affected by hundreds of large gully systems, varying in extent from somewhat < 200 m2 to > 3 ha. A study set of 90 gully systems were vectorized from aerial ortophotos and analysed in relation to a set of potential control variables, obtained by reconstructing the pre-erosion topography. Multiple linear regression and logistic regression were combined in order to investigate controls over the general morphometry of these landforms (area, perimeter and sinuosity) and their location in the respective basins. Results indicate that gully systems initiated as bank gullies in response to an incision by the drainage network in both basins and then evolved as a function of upslope contributing area, developing progressively more sinuous outlines as a result of headcut bifurcation. Pre-erosion topographic form was shown to have only a marginal effect, with results suggesting gully systems have grown essentially through mass movements in headcuts and sidewalls, influenced by subsurface water.