Po River (Virginia)

The Po ( , ; la, Padus or ; Ancient Ligurian: or ) is the longest river in Italy. It flows eastward across northern Italy starting from the Cottian Alps. The river's length is either or , if the Maira, a right bank tributary, is included. The headwaters of the Po are a spring seeping from a stony hillside at Pian del Re, a flat place at the head of the Val Po under the northwest face of Monviso. The Po then extends along the 45th parallel north before ending at a delta projecting into the Adriatic Sea near Venice.

It is characterized by its large discharge (several rivers over 1,000 km have a discharge inferior or equal to the Po). It is, with the Rhône and Nile, one of the three Mediterranean rivers with the largest water discharge. As a result of its characteristics, the river is subject to heavy flooding. Consequently, over half its length is controlled with embankments.

The river flows through many important Italian cities, including Turin, Piacenza, Cremona and Ferrara. It is connected to Milan through a net of channels called '' navigli'', which Leonardo da Vinci helped design. Near the end of its course, it creates a wide delta (with hundreds of small channels and five main ones, called ''Po di Maestra'', ''Po della Pila'', ''Po delle Tolle'', ''Po di Gnocca'' and ''Po di Goro'') at the southern part of which is Comacchio, an area famous for eels. The Po valley was the territory of Roman Cisalpine Gaul, divided into Cispadane Gaul (south of the Po) and Transpadane Gaul (north of the Po).

Geography

The Po has a drainage area of 74,000 km² in all, 70,000 of those being in Italy, of which 41,000 is in montane environments and 29,000 on the plain. The slope of the Po's river valley decreases from 0.35% in the west to 0.14% in the east, a low gradient. Along its path lie 450 standing lakes. Almost all of the rest of the non-Italy basin is in Switzerland, primarily in the canton of Ticino, which is essentially drained by the Ticino river rising in the Gotthard Area, and includes Lake Maggiore and Lake Lugano. A small part of Grisons canton drains to the Po, partly via the Ticino. The Simplon Valley in the canton of Valais is drained by the Diveria.

A minute section of the Po basin belongs to France in the Valle Etroite (literally, the Narrow Valley) running from Mont Thabor to the Italian ski resort of Bardonecchia. Valle Etroite is so remote it is essentially administered by Italy (telephone network, rubbish collections etc.). Further minuscule parts of the Po's basin (measurable in the hundreds of metres of linear distance) within France are found in the form of small streamheads forced into France by the 1947 Peace Treaty of Paris as a punitive measure against Italy. These can be found on the Mont Cenis and Mongenevre passes. The former contains a reservoir dammed at the Po end and so technically constitutes part of its basin, although it contributes little to the water flow as the water is, by definition, retained by the dam. The Po is the longest river in Italy; at its widest point its width is .

Po Valley

The vast valley around the Po is called the Po Basin or Po Valley (Italian ''Pianura Padana'' or ''Val Padana''); the main industrial area and the largest agricultural area in the country - accounting for 35% of Italian agricultural production. In 2002, more than 16 million people lived in the area, at the time nearly one-third of the population of Italy.

The two main economic uses of the valley are for industry and for agriculture. The industrial centres, such as Turin and Milan, are located on higher terrain, away from the river. They rely for power on the numerous hydroelectric stations in or on the flanks of the Alps, and on the coal/oil power stations which use the water of the Po basin as coolant. Drainage from the north is mediated through several large, scenic lakes, commonly referred to as the Italian Lakes, and shared with Switzerland. The streams are now controlled by so many dams as to slow the river's sedimentation rate, causing geologic problems. The expansive, moist and fertile flood plain is reserved mainly for agriculture and is subject to flash floods, even though the overall quantity of water is lower than in the past and lower than demand. The main products of the farms around the river are cereals including – unusually for Europe – rice, which requires heavy irrigation. The latter method is the chief consumer of surface water, while industrial and human consumption use underground water.

Tributaries

The Po has 141 tributaries. They include (R on the right bank, L on the left, looking downstream):

* Pellice (L)
* Varaita (R)
* Maira (R)
* Chisola (L)
* Sangone (L)
* Dora Riparia (L)
* Stura di Lanzo (L)
* Malone (L)
* Orco (L)
* Dora Baltea (L)
* Stura del Monferrato (R)
* Sesia (L)
* Rotaldo (R)
* Grana del Monferrato (R)
* Tanaro (R)
* Scrivia (R)
* Agogna (L)
* Curone (R)
* Staffora (R)
* Ticino (L)
* Versa (R)
* Tidone (R)
* Lambro (L)
* Trebbia (R)
* Nure (R)
* Adda (L)
* Arda (R)
* Taro (R)
* Parma (R)
* Enza (R)
* Crostolo (R)
* Oglio (L)
* Mincio (L)
* Secchia (R)
* Panaro (R)

The Reno (R) was a tributary of the Po until the middle of the eighteenth century when the course was diverted to lessen the risk of devastating floods. The Tanaro is about 50 km (31 mi) longer than the upper Po at their confluence near Alessandria.

Po Delta

Protected areas

The Po Delta wetlands have been protected by the institution of two regional parks in the regions in which it is situated: Veneto and Emilia-Romagna. The Po Delta Regional Park in Emilia-Romagna, the largest, consists of four parcels of land on the right bank of the Po and to the south. Created by law in 1988, it was managed by a consortium, the ''Consorzio per la gestione del Parco'', to which Ferrara and Ravenna provinces belong as well as nine comuni: Comacchio, Argenta, Ostellato, Goro, Mesola, Codigoro, Ravenna, Alfonsine, and Cervia. Executive authority resided in an assembly of the presidents of the provinces, the mayors of the comuni and the board of directors. They employed a Technical-Scientific Committee and a Park Council to carry out directives. In 1999 the park was designated a World Heritage Site by UNESCO and was added to "Ferrara, City of the Renaissance, and its Po Delta." From 2012 the park is managed by the ''Ente di Gestione per i Parchi e la Biodiversità - Delta del Po'', composed by the ''comuni'' of Alfonsine, Argenta, Cervia, Codigoro, Comacchio, Goro, Mesola, Ostellato
and Ravenna. The of the park contain wetlands, forest, dunes and salt pans. It has a high biodiversity, with 1000–1100 plant species and 374 vertebrate species, of which 300 are birds.

Active delta

The most recent part of the delta, which projects into the Adriatic between Chioggia and Comacchio, contains channels that connect to the Adriatic and therefore is called the active delta by the park authorities, whereas the fossil delta contains channels that no longer connect the Po to the Adriatic (but once did). The active delta was created in 1604 when the city of Venice diverted the main stream, the ''Po grande'' or ''Po di Venezia'', from its channel north of Porto Viro to the south of Porto Viro in a channel then called the ''Taglio di Porto Viro'', "Porto Viro cut-off". Their intent was to stop the gradual migration of the Po toward the lagoon of Venice, which would have filled up with sediment had contact been made. The subsequent town of Taglio di Po grew around the diversionary works. The lock of ''Volta Grimana'' blocked the old channel, now the ''Po di Levante'', which flows to the Adriatic through Porto Levante.

Below ''Taglio di Po'' the ''Parco Regionale Veneto'', one of the tracts under the authority of the ''Parco Delta del Po'', contains the latest branches of the Po. The ''Po di Gnocca'' branches to the south followed by the ''Po di Maestra'' to the north at Porto Tolle. At Tolle downstream the ''Po di Venezia'' divides into the ''Po delle Tolle'' to the south and the ''Po della Pila'' to the north. The former exits at Bonelli. The latter divides again at Pila into the ''Busa di Tramontana'' to the north and the ''Busa di Scirocco'' to the south, while the mainstream, the ''Busa Dritta'', enters ''Punta Maistra'' and exits finally past Pila lighthouse.

Despite the park administration's definition of the active delta as beginning at Porto Viro, there is another active channel upstream from it at Santa Maria in Punta, where the ''Fiume Po'' divides into the ''Po di Goro'' and the ''Po di Venezia''.

Fossil delta

The fossil Po is the region of no longer active channels from the Po to the sea. It begins upstream from Ferrara. The ''Fiume Po'' currently flowing to the north of Ferrara is the result of a diversion at Ficarolo in 1152 made in the hope of relieving flooding in the vicinity of Ravenna. The diversion channel was at first called the ''Po di Ficarolo''. The ''Fiume Po'' before then followed the ''Po di Volano'', no longer connected to the Po, which ran to the south of Ferrara and exited near Volano. In Roman times it did not exit there but ran to the south as the ''Padus Vetus'' ("old Po") exiting near Comacchio, from which split the ''Po di Primaro'' exiting close to Ravenna.

Before 1152 the seaward extension of today's delta, about , did not exist. The entire region from Ravenna to Chioggia was dense swamps, explaining why the Via Aemilia was constructed between Rimini and Piacenza and did not begin further north.

Geologic history

The Mediterranean Basin is a depression in the Earth's crust caused by the African Plate slipping under the Eurasian Plate. Typically in geologic history the depression is filled with sea water under various geologic names such as Tethys Sea. In the last period of the Miocene Epoch, the Messinian (7–5 mya), the Messinian salinity crisis, a near drying of the Mediterranean, was caused by the sea level dropping below the sill at the Strait of Gibraltar and the equilibrium between evaporation and replenishment shifting in favor of evaporation. At that time the Po Valley and the Adriatic depression were a single canyon system thousands of feet deep. On the southwest the Apennine Mountains bordered a land mass termed Tyrrhenis geologically. Their orogeny was just being completed in the Miocene. On the north the Alpine Orogeny had already created the Alps.

At the end of the Messinian the ocean broke through the sill and the Mediterranean refilled. The Adriatic transgressed into all of northern Italy. In the subsequent Pliocene

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