Recent gully erosion in the El Cautivo badlands (Tabernas, SE Spain)

Derailed geomorphic observations in El Cautivo badland area, in the semiari d SE of Spain, show a recent renewal of linear channel erosion following a period of aggradation. Prior to this renewal, the landscape was characteriz ed by smooth colluvial pediments on the north-east facing aspects and relat ively steep crusted microhillslopes on south-west facing aspects. First-ord er valleys within the badland landscape were smooth-floored and filled with silty deposits connected with pediment colluvial deposits. The conditions under which valley bottom deposition was replaced by channel Incision were analyzed by a study of flow hydraulics (magnitude-frequency of eroding even ts vs. resistance of valley floor to water flow) and the pollen sequence in the recently cut deposits. Changes in the vegetation-carpeting the valley bottom seem to be responsible fur the deposition-erosion alternation rather than changes in the magnitude-frequency of major runoff events, given the high sensitivity of flow hydraulics to changes in roughness. Present rainfa ll rates would not, on their own, be able to erode a well-vegetated valley bottom. Yet, the pollen sequence in the valley Fills demonstrates a progres sive deterioration in climatic conditions with the eventual disappearance o f groups that require a constant level of soil moisture (Alnus, Corylus, Cy peraceae), An increase in yearly rainfall or a change in temporal patterns would increase infiltration leading to permanent denser vegetation on foots lopes and valley floors, helped by the natural water harvesting system. Thi s vegetation cover would reduce the flow velocity and induce the deposition of the colluvium and valley fills, whereas erosion on headslopes would hav e been active enough to supply the sediments that were deposited downslope or on the valley bottoms. The reactivation of linear erosion would seem, th erefore, to be a consequence of the decay of this permanent vegetation, ind uced by an increase in the duration or severity of drought periods, which m eant that the water storage capacity of the valley fillings was insufficien t to support it. (C) 2000 Elsevier Science B.V. All rights reserved.