The Ribagorda sand gully (east-central Spain): Sediment yield and human-induced origin

Abstract

Gullies are developed under different climatic conditions and lithologies; however, those formed on sands have been scarcely described. This paper reports the study of the Ribagorda sand gully, 2.57 ha in area (east-central Spain). The main objectives were to characterize and quantify its geomorphic dynamics and to trace its origin. We described the landforms of the gully and measured the surface strength of the sand. We monitored, for six years, the filling of the storage areas of three check dams built downstream from the gully, and related it with rainfall characteristics. We also described the nature of the sediments trapped by the dams and estimated the amount of sediment eroded since the gully formation. Finally, we consulted historical records and maps to determine past land uses and transformations that may have affected the origin of the gully. The study shows a high diversity of landforms, denoting active processes, consistent with a measured mean annual sediment yield of 114 Mg ha−1 yr−1. A statistically significant relationship exists between the mass of sediment (Mg) and: 1) the total rainfall (mm) (P = 0.0007) or 2) the analysed rainfall intensities. Among five identified facies in the sedimentarywedge, the sandy ones are predominant. The total amount of sediment eroded by the Ribagorda gully since its originwas 962,800Mg. The results are unequivocal signs of an intense geomorphic activity within the gully, with an alluvial-fan type deposition in the dams.We interpret that the Ribagorda gully was initiated by deforestation after the 13th century, when forests began to be intensively logged, and before the 18th century,when the gullywas first indirectly described in print. The age, origin, evolution and dynamics of this gully indicate that this landscape is currently evolving towards a new steady state, after human disturbances over centuries. Given the gully evolution and local extent, we suggest that no correction measures are needed for its management.