Po Valley


The Po Valley, Po Plain, Plain of the Po, or Padan Plain is a major geographical feature of northern Italy. It extends approximately in an east-west direction, with an area of including its Venetic extension not actually related to the Po basin; it runs from the Western Alps to the Adriatic Sea. The flatlands of Veneto and Friuli are often considered apart since they do not drain into the Po, but they effectively combine into an unbroken plain, making it the largest in Southern Europe. It has a population of 17 million, or a third of Italy's total population.
The plain is the surface of an in-filled system of ancient canyons extending from the Apennines in the south to the Alps in the north, including the northern Adriatic. In addition to the Po and its affluents, the contemporary surface may be considered to include the Savio, Lamone and Reno to the south, and the Adige, Brenta, Piave and Tagliamento of the Venetian Plain to the north, among the many streams that empty into the north Adriatic from the west and north.
Geo-political definitions of the valley depend on the defining authority. The Po Basin Water Board, authorized in 1989 by Law no. 183/89 to oversee "protection of lands, water rehabilitation, the use and management of hydro resources for the national economic and social development, and protection of related environment" within the Po basin, has authority in several administrative regions of north Italy, including the plain north of the Adriatic and the territory south of the lower Po, as shown in the regional depiction included with this article. The law defines the Po basin as "the territory from which rainwater or snow and glacier melt flows on the surface, gathers in streams of water either directly or via tributaries...". The United Nations Environment Program includes the Alps and Apennines as far as the sources of the tributaries of the Po but excludes Veneto and that portion of Emilia-Romagna south of the lower Po; that is, it includes the region drained by the Po but only the Po and its tributaries.
The altitude of the valley through which the Po flows, exclusive of its tributaries, varies from approximately below sea level in the Polesine subregion to about at the river's origin in the southern Piedmontese province of Cuneo, also known as the Provincia granda. The valley is crossed by a number of affluents running down from the Alps in the north and from the Apennines in the south. The Po's major affluents include the Tanaro, Scrivia, Trebbia, Panaro and Secchia in the south, Dora Riparia, Dora Baltea, Sesia, Ticino, Lambro, Adda, Oglio and Mincio in the north.

Geology

The Po Valley and the Adriatic overlay a foreland basin and a system of deeply buried ancient canyons surviving from the tectonic collision of an offshore land mass, Tyrrhenis, with the mainland, an incident within the collision of the African and Eurasian plates. Since the Messinian the system has been filling with sediment mainly from the older Apennines but also from the Alps. The shoreline of the Adriatic depends on a balance between the sedimentation rate and isostatic factors. Until about 1950, the Po delta was prograding into the Adriatic. After that time due to human alteration of geologic factors, such as the sedimentation rate, the delta has been degrading and the coastline subsiding, resulting in ongoing contemporaneous crises in the city of Venice.
The Malossa gas condensate field was discovered in 1973 and produces at depths of from the Upper Triassic Dolomia Principale dolomite and the Lower Jurassic Zandobbio dolomite, capped by the Lower Cretaceous Marne di Bruntino marl.

Geography

The Po Valley is often regarded as a syncline, or dip in the crust due to compression at the edges. Regardless of whether this concept accurately describes its geology, the valley is manifestly a sediment-filled trough, or virtual syncline, continuous with the deeps of the Adriatic Sea. The surface terrain is therefore divided into two overall types of landform: the plain, or flat surface of the fill, and the anticline at the edges, taking the form of hilly country in which the outcrops of the original rock are visible along with alluvial fans formed from the outwash of the more severe anticlinal terrain; that is, the Apennines and the Alps.
The valley is broadly divided into an upper, drier part, often not particularly suited for agriculture, and a lower, very fertile, and well-irrigated section, known in Lombardy and western Emilia as la Bassa, 'the low '. The upper areas of the Po valley take local names which reflect in their meanings their being modestly suited for farming. So we have the Piedmontese vaude and baragge, the Lombard brughiere and Groane, or, exiting from the Po valley proper, the Friulian magredi, areas remote from easily reachable water tables and covered with dense woods or dry soils.
This specific meaning for 'lower plain' derive from a geologic feature called the fontanili line or zone, a band of springs around the Val Po, heaviest on the north, on the lowermost slopes of the anticline. It varies from a few kilometres to as much as wide. The fontanili line is the outcrop, or intersection, of the anticline's water table with the surface at the edge of the bassa. The rock above the line is porous. Surface water in the intermittent streams of the mountains tends to disappear below ground only to spring out again in the spring zone. The spring zone is often called "the middle valley."
Surface runoff water is not of much value to the valley's dense population for drinking and other immediate uses, being unreliable, often destructive, and heavily polluted by sewage and fertilizers. Its main anthropic value is for hydro-electric power, irrigation, and industrial transport. The cost of purifying it for human consumption makes that process less feasible. The fresh drinking water comes from hundreds of thousands of wells concentrated especially in the fontanili zone. The major settlements, therefore, are also in that zone, which has become the centre of economic development and industry in Italy, and now is an almost continuous megalopolis stretching from Turin to Trieste.
The bassa Padana was settled and farmed earliest, in Etruscan and Roman times. After the collapse of the Roman Empire, lack of maintenance of the irrigation systems associated with a cooling climate phase led to the progressive waterlogging of the Po Valley and the natural depressions on the right side of the Po turned in vast swamp basins. The waterlogging process of the area continued until the 10th-century influencing the human sustenance and settling practices. According to historical-archaeological data, indeed, the wetlands were exploited for fishing as well as for transport by boat while the early medieval sites settled on the fluvial ridges, in topographically higher and strategic position in the surrounding swampy meadows.
The Po Valley has been completely turned to agriculture since the Middle Ages, when efforts from monastic orders, feudal lords and free communes converged. The older and smaller cities deriving from ancient times are still located there.
According to historical maps and documents the land reclamation of the Po Valley reached its peak during the Renaissance and continued in the Modern Age, with the last marsh areas only being reclaimed in the 20th century: channels and drainage system are still active and allow the Po Valley to be drained and be cultivatable.

Tributaries

Major cities

From upstream to downstream:

Piedmont

The Po Valley has a generally humid subtropical climate. The conformation of the plain, surrounded by the Alps and the Apennines, and the influence of the Adriatic Sea cause high levels of relative humidity throughout the year. The climate is increasingly warmer and more humid farther south and east.
Winters are cool and damp, with January mean temperatures ranging between. Fog and mist are frequent, although the urban heat effect, decreased air pollution and climate change have made winters less foggy and cold than before. Snow and winter droughts can deny sufficient moisture to the soil for agriculture. Summers are hot and humid, with July mean temperatures ranging between . Frequent thunderstorms and sudden hailstorms can produce large hail, dump large quantities of rain, and destroy crops. Supercell thunderstorms produce large hail, with significant agricultural costs. Tornadoes are common in the plains of the valley. The Friuli-Venezia Giulia, beyond the Po plain itself, is downwind of the mountains and upwind of moisture sources from nearby is an exception. Spring and autumn are well-marked and pleasant. Both winter and summer are less mild in the lower parts along the Po, while the Adriatic Sea and the great lakes moderate the local climate in their proximity.
File:Po Valley, Italy.jpg|thumb|center|upright=4|The Po Valley as seen by the ESA's Sentinel-2
Rainfall varies between and equally distributed during the year. Rainfall maximums are during autumn and spring. Winds are usually weak, although sudden bursts of foehn or thunderstorms can sweep the air clean. The almost enclosed nature of the Padan basin with much road traffic, makes it prone to a high level of pollution in winter, when cold air clings to the soil. The natural potential vegetation of the Po basin is a mixed broadleaf forest of pedunculate oak, poplars, European hornbeam, alders, elderberry, elms, willows, maples, ash, and other central-European trees. The largest remaining fragments of forest are located around the River Ticino and are protected by a Biosphere Reserve.