The cuneo salino in the Po Delta

The Po Delta

Northern Italy is characterized by an extensive alluvial plain, the Po Valley, which is crossed by the Po River, the longest river in the country. The river originates from the Monviso massif in the Piedmont region and flows eastward for over 650 kilometers before entering the northern Adriatic Sea. At its end, the Po forms a deltaic system composed of multiple distributary branches. This area, known as the Po Delta, represents one of the most significant wetland complexes in the Mediterranean region, and it extends across approximately 400 km².

From a geomorphological perspective, the Po Delta is shaped by both natural and anthropogenic factors. Processes such as sediment deposition, land subsidence, and coastal erosion interact with human interventions for water management, land reclamation, and coastal defense, continuously modifying the delta’s morphology and hydrological balance.

The Po Delta hosts one of the largest clusters of Natura 2000 sites in Italy, designated under the Habitats Directive (92/43/EEC) and the Birds Directive (2009/147/EC). These sites safeguard both habitats of community interest, such as coastal lagoons, reedbeds, and riparian forests, and species of conservation concern, particularly migratory and nesting bird populations.

The Cuneo Salino

In the Po River, the phenomenon of the cuneo salino poses a growing threat to the biodiversity of the deltaic system. By definition, it is a hydrological phenomenon that occurs in river mouths and estuaries where freshwater and seawater meet. Due to the difference in density between freshwater and saltwater, the latter tends to advance upstream along the riverbed, forming a wedge-shaped intrusion. Freshwater, being lighter, flows above the denser marine water, while the saline mass slowly penetrates inland at the bottom of the channel.

This phenomenon alters the composition of aquatic communities, endangers species adapted to low salinity, and disrupts the ecological balance of wetlands, lagoons, and riparian zones that depend on the delicate gradient between fresh and marine waters.

The extension of the salt wedge depends on several factors, including river discharge, tidal oscillations, and sea level. When river flow decreases, especially during droughts or in summer months, the wedge can move farther upstream; conversely, high river discharge pushes the saline intrusion back towards the sea.

Over the course of a year, the balance between river discharge and saline intrusion draws a fluctuating line. Periods of abundant flow strengthen the river’s resistance, keeping saltwater at bay, while prolonged low flows open the way for the sea to advance inland. This alternation outlines a pattern that reveals both the natural rhythms of the Po and the increasing vulnerability of the delta to climatic extremes.

To monitor the affected area, a fixed network of multiparametric probes has been installed in the Po Delta, in agreement with local authorities and ARPAV. These nine stations are distributed across key lagoons and record in real time the main physico-chemical parameters. The continuous acquisition of data allows early detection of environmental disturbances that could compromise aquaculture resources and freshwater availability. In addition to scientific research, the monitoring system provides essential information for water management, agriculture, and the protection of wetland ecosystems threatened by the inland progression of the cuneo salino.

Biodiversity endangers

Brackish water intrusion threatens the survival of fish populations that rely on freshwater for critical life stages, such as spawning and juvenile development. Amphibians, including frogs and newts, suffer from the degradation of freshwater ponds and marshes, while bird species that depend on these habitats for feeding and breeding—such as herons, bitterns, and various ducks—lose key ecological niches.

Equally important is the impact on riparian and aquatic vegetation that are highly vulnerable to increased salinity. Consequently, their decline reduces habitat quality for invertebrates, amphibians, and fish, and accelerates the ecological shift towards communities dominated by halophytic plants which are typical of salt marshes but unsuitable for maintaining the freshwater biodiversity of the delta.

The deterioration of freshwater habitats also weakens the resilience of Natura 2000 sites in the Po Delta, many of which are designated for the protection of priority habitats like alluvial forests, riparian wetlands, and coastal lagoons. The reduction in river discharge exacerbates the problem, as lower water availability facilitates the inland progression of the salt wedge. As a result, the cuneo salino not only endangers individual species but also undermines the integrity of entire ecological communities, threatening one of Europe’s most important biodiversity reservoirs.