An important problem that teachers of geology have to face up to day by day is to make the students to understand geomorphological and sedimentary processes that developed in past times along thousands or millions of years and in very large regions of the Earth surface. In order to fight this deficiency, which appears particularly in initiation students, it turns out very useful to analyze field examples where these phenomena happen to a human time scale and in small sized sedimentary basins.
An analysis has been made of sedimentary systems involved in the rapid silting of a reservoir constructed in 1974 in Alhama de Granada (S. Spain); in only 30 years the storage capacity of the reservoir has shrunk by 80% and its perimeter has decreased by 64%. A study of sediment lithofacies identified in a series of shallow trenches and of georadar facies identified in a series of almost 900m lines of Ground Penetrating Radar (GPR) images, together with a survey of surface geology, has identified three alluvial systems whose distal deltas have filled in the reservoir. These deltas correspond to the longitudinal river drainage of the Alhama River (LS), to the transverse drainage system whose source is the detrital sediments of the Granada Basin infill (DTS), and to the transverse system nourished by the crystalline, metamorphic reliefs of the Alpujárride Complex (Internal Zone of the Betic Cordillera) (CTS).
The 13 lithofacies observed in the trenches dug in the deltas reveal channel infill and overflow, sheet and sediment gravity flows, and settling and pedogenesis of fine sediments in interdistributary bay zones. The GPR profiles indicate that each of the three deltas has had an independent history of build-up in which progradation stages have alternated with vertical accretion; there have also been greater and lesser periods of asynchronous sedimentation in the three deltas.
Thus, there are three phases in the evolution of the reservoir siltation: 1) an initial stage (1974–1977) typified by northward progradation of the LS delta of about 100 m yr-1 and an eastward progradation of the DTS delta of about 20 m yr-1; 2) an intermediate stage (1977–1984) in which the LS progradation slowed to 25 m yr-1 and the axial drainage became obstructed due to the considerable eastward progradation of the DTS delta; and 3) a final phase (1984–present) in which there have been few changes in the areal distribution of the deltas apart from a southeastward expansion of the DTS delta. Generally, aggradational growth patterns have dominated in this final phase.
The high velocity of the geomorphologic evolution of this small area together with the possibility of combination of field data and geophysics to understand the dynamics of sedimentary infilling of this basin, give to this example a very high didactic potential in the teaching of geomorphology, stratigraphy, sedimentology and geological hazards.