Po (river)

The Po (/poʊ/ POH, Italian: [ˈpɔ]) is the longest river in Italy. It flows eastward across northern Italy, starting from the Cottian Alps. The river's length is 652 km (405 mi), or 682 km (424 mi) if the Maira, a right bank tributary, is included. The headwaters of the Po are formed by 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.

Draining a basin of 74,000 km² (29,000 sq mi), the Po 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).

The Po has a drainage area of 74,000 km² (29,000 sq mi) in all, 70,000 km² (27,000 sq mi) of those being in Italy, of which 41,000 km² (16,000 sq mi) is in montane environments and 29,000 km² (11,000 sq mi) 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 river Ticino rising in the Gotthard Area, and includes Lake Maggiore and Lake Lugano. A small part of the canton of Grisons 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 Vallée Étroite [it; fr] (literally, the Narrow Valley) running from Mont Thabor to the Italian ski resort of Bardonecchia. Although in France, Vallée Étroite 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 it is 503 m (1,650 ft) across.

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.

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

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.