Geological processes in the Anthropocene: the Po River Delta

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Geological processes in the Anthropocene: the Po River Delta

Marit B. Brommer

Delft University of Technology

Po Delta Evolution

The Po Delta is one of the best documented deltas of the western world (McManus, 2004). The delta region of the Po River has been inhabited since the Etruscans founded the ancient port Adria at about 1000 B.C (Fig. 1). The Etruscans tried to control the river and to protect their port from silting up. Ultimately they had to give up and lost their fight (Nelson, 1970). From 1000 BC on the Po Delta prograded seaward while the main distributary channel shifted to the south through Ferrara (Po di Ferrara). Two major distributaries (Po di Primaro and Po di Volano) entered the Adriatic south of the modern delta.

The Po di Volano was active until 1200 A.D. The most ancient descriptions of the Po Delta come from Polybius (150 B.C.), Strabo (66 B.C.- 21 A.D.) and Pliny the Elder 65-75 A.D (Naturalis Historia). Pliny the Elder described seven river outlets in the ancient Volano delta that were used by the Romans in the 1st century A.D. At that time the flow from the distributaries was dispersed along a wide front through small outlets with a cuspate morphology (Nelson, 1970). The cuspate deltas were wave-dominated and advanced with an average rate of 4 m/yr.

In 1150 AD a major natural diversion of the river took place at Ficarolo. Attempts to protect the harbor of Ferrara were insufficient, and the new channel (Tramontana) flowed northeast and formed an active delta that advanced at a rate of 25 m/yr. After 1600 A.D. the Po Delta started to become a human-controlled environment. The new Tramontana channel started to endanger the Venetian Republic by silting up the lagoon of Venice. In 1604 the Venetians dug an artificial canal at Porto Viro and diverted the main channel southward. The canal forced the river to advance in a southeasterly direction at a rate of 86 m/yr. In 1840 the eastern distriburary Po della Pila became active. The average progradation rate was on average 50 m/yr (Visentini, 1940). According to Ciabatti (1967) the Po delta advanced during the last 400 years much more quickly than before, most likely due to an increase in sediment supply as a result of deforestation. Yet, Nelson (1970) argues that the rapid advancement of the Po Delta from 1600 AD onwards was primarily caused by the confining the Po River discharge to a small area, which transformed the Delta from a cuspate to a lobate, supply-dominated, morphology.

Present-day Po Delta

At present, the Po Delta consist of five major distributaries; Goro, Gnocca, Tolle, Pila and Maestra. These distributaries all have artificial levees preventing (natural) sedimentation of the delta plain (Correggiari et al., 2005). Consequently, the sediment fines are directly discharged into the Adriatic Sea. According to Dal Cin (1983) the present-day Po delta is undergoing retreat and is evolving towards a wave-dominated cuspate morphology. This suggests that the Po Delta is transforming back into its natural, perhaps preferential, morphology as it was before 1600 AD.

References

McManus, J. (2004). Deltaic responses to changes in river regimes. Marine Chemistry, 79, 155-170.

Nelson, B.W. (1970). Hydrography, sediment dispersal, and recent historical development of the Po river delta, Italy. SEPM Sp. Publ., 15, 152-184.

Visentini, M., Borghi, G. (1938). Le spiagge padane. Ricerche sulle variazioni delle spiagge italiane. CNR Report, Roma, 7, 137 p.

Ciabatti, M. (1967). Richerche sull’evoluzione del delta padano. G. Geol. 34, 381-410.

Correggiari, A., Cattaneo, A., and F. Trincardi (2005). The modern Po Delta system: lobe switching and asymmetric prodelta growth. Marine Geology, 222-223, 49-74.

Dal Cin, R. (1983). I littorali del delta Po e alle foci dell’Adige e del Brenta: caratteri tessiturali e disperzione dei sedimenti, cause dell’arretramento e previsioni sull’evoluzione future. Boll. Soc. Geol. Ital. 102, 9-56.