From the Geomorphic Process to Basin Architecture: Anatomy of the Infill of an Alluvial-Lacustrine System in Southern Spain

The heavy rains in the winter of 2009-2010 in Spain activated all the sediment-feeder systems of a small reservoir built in the 1970s north of the Sierra de Almijara (province of Granada). The intensity of this activity has aided in recognizing a series of geomorphic features allowing the re-interpretation of previous data on the subsoil obtained from Ground Penetrating Radar (GPR) and sedimentological analysis involving the study of a series of shallow trenches dug in various parts of the basin. The unequal influence of three feeder systems in the process of silting up this small artificial lake has been noted. The main contributor is the longitudinal axial drainage system, which is building up a huge delta whose progradation and aggradation dynamics are strongly influenced by the obstruction of a transverse delta fan on a highly erodible source area comprising Tertiary detrital sediments. Far less important in the construction of the stratigraphic architecture are the transverse fans lying against the Palaeozoic to Triassic metamorphic basement. The most notable geomorphological change over the last 35 years has been in the longitudinal axial system. Over a period of only eight years (1977-1984), it morphed from a coarse-grained braided fluvial system with a single channel in which longitudinal bars and thalwegs constantly changed position with every flood to a multichannel anastomosed system characterized by interwoven narrow, sinuous channels. Many of these channels are only covered during floods, and they are separated by heavily vegetated islands that act as traps for the overbank deposits. The difference in soil uses in the different parts of the basin also plays an important role, causing the progradation rate of the deltas to range widely, from 0 all the way to 100 m yr. This study reveals that in countries of the Mediterranean region torrential rains, the scanty vegetation cover, and the alteration of the longitudinal profile of dammed rivers interact causing extreme siltation of reservoirs. Under these situations it is extremely important to study the dynamics of mass movement in the parts of the basin with different lithology to carry out effective plans of management of the reservoir and to minimize the possibility of environmental and economic negative impacts.