Spatial distribution of gully head activity and sediment supply along an ephemeral channel in a Mediterranean environment

In this study, we examined the factors that control the spatial distribution of bank gully heads along a reach of an ephemeral river (Rambla Salada) in an area threatened by desertification in Southeast Spain. The activity of 458 gully heads was assessed in the field by pre-defined criteria such as sharp edges, presence of a plunge pool, tension cracks, recent deposited sediments, flow marks, and vegetation re-growth. The results showed that land use has a significant impact on bank gully head activity. Recent land-use changes involving the extension of almond cultivation appears to intensify bank gully head activity. Also, lithology has a clear impact on the bank gully extension. It was further investigated whether the gully heads were important sediment sources that contributed to reservoir sedimentation. The density of (very) active bank gully heads along the study reach was one per 17 m of channel length. Average annual retreat volumes were derived from measurements at 46 active gully heads (4.0 m(3) y(-1)). By selecting all the channel sections in the catchment of the Puentes Reservoir with a similar pattern of bank gullies using aerial photographs, an estimate of basin-wide sediment production of bank gully heads was established. It was estimated that the retreat of active bank gully heads alone in the 12,760 ha study area (representing 12% of the total catchment area of the Puentes Reservoir) produced 6% of the sediment filling up the reservoir. Considering that the sediment is also derived from other sources such as channel walls, channel beds, and hillslopes, the overall conclusion is that bank gully expansion in Southeast Spain is a major point source of sediment and therefore, a major process of land degradation. (C) 2000 Elsevier Science B.V. All rights reserved.