River-mouth geomorphological changes over >130 years (1882–2014) in a small Mediterranean delta: Is the Magra delta reverting to an estuary?

The morphological and volumetric changes of the mouth of the small Magra River (catchment size: 1400 km2) in the Western Mediterranean were analyzed from high-resolution bathymetric data collected during several oceanographic surveys from 1882 to 2014, processed following international hydrographic standards (IHO S-44), and stored in the Italian Hydrographic Institute database. The data were collected using lead lines, single-beam and multibeam acoustic systems with the same standard of accuracy. The morphological history of the seabed fronting the river mouth and of the adjacent coastal zone was reconstructed using generated bathymetric Digital Terrain Models (DTMs) reflecting different timescales (secular, multi-decadal, decadal). The results highlight changes in the geometry of the Magra River mouth and of the coastal profile and bottom features that have accompanied variations in the sediment budget at the afore-mentioned timescales. The mouth of the Magra, like that of many rivers in the Mediterranean, evolved over the last 2–3000 years into a delta from a primitive estuarine embayment. Budget computations obtained from comparison of the DTMs show that the river mouth and adjacent delta-front area have lost sediment to the tune of about 3 Mm3 over 1882-2000, of which 1.8 Mm3 concern 1954-2000 and 1 Mm3 were lost in the period 2000-2014 inside the terminal river part. This trend is characterized by a transformation from what appeared as a river-dominated delta into an increasingly wave-dominated and sediment-depleted river mouth. The mouth of the Magra is increasingly exhibiting a morphology typical of an estuary, in the geomorphological and geological senses, as defined, for instance, by Dalrymple et al. (1992), notwithstanding the microtidal context of the Magra River. This forced reversal towards the primitive estuarine state is attributed to the severity of sediment depletion at the mouth, notably caused by massive extraction of bed sediments for road construction in the 1960s to 1980s, and dredging since 2000. The trend appears to be further reinforced by constrained lateral channel migration on the northern side of the river due to the existence of the Punta Bianca promontory. Many deltas in the Mediterranean and elsewhere are now being seriously affected by a decrease in fluvial sediment supply due to human activities. The example of the Magra, where massive modern sediment withdrawal has led to river-mouth geomorphic reversal to a more primitive estuarine state that initially characterized this delta, probably illustrates the potential fate of increasingly sediment-depleted deltas in the Mediterranean and elsewhere.