PO (anc. Padus, Gr. Πάδος), a river of northern Italy, and the largest in the whole country, with a total length of about 310 m. direct from the source to the mouth, but, including its many windings, of some 417 m. The navigable portion from Casale Monferrato to the mouth is 337 m., the minimum width of this portion 656 ft., and its minimum depth 7 ft. Owing to the prevalence of shallows and sandbanks, navigation is difficult.
The Po is the dominating factor in north Italian geography, north Italy practically consisting of the Po basin, with the surrounding slopes of the Alps and Apennines. For a description of its course, and a list of its principal tributaries see Italy. The area of its basin, which includes portions of Switzerland and Austria, is estimated at 26,798 sq. m.
In the first 21 m. of its course, down to Revello (west of Saluzzo), the Po descends no less than 5250 ft., or a fall of 47.3 : 1000, forming a very remarkable contrast to its fall lower down. From the confluence of the Ticino its fall is about 0.3 : 1000; from the beginning of the delta below Ferrara, 0.08 : 1000. At Turin it has an average width of 400 to 415 ft., a mean depth of 312 to 512 ft., and a velocity of 1 to 3 ft. in the second. The mean depth from the confluence of the Ticino (altitude 217 ft.) downwards is 6 to 15 ft. The river is embanked from Piacenza, and continuously from Cremona, the total length of the embankments exceeding 600 m. Owing to its confinement between these high banks, and to the great amount of sedimentary matter which the river brings down with it, its bed has been gradually raised, so that in its lower course it is in many places above the level of the surrounding country. A result of confining the stream between its containing banks is the rapid growth of the delta. Lombardini calculated that the annual increase in the area of the Po delta during the period 1300 to 1600 amounted to 127 acres; but during the period 1600 to 1830 it rose to 324 acres. Marinelli[1] estimated that between the years 1823 and 1893 the annual increase was at the average rate of 173 to 175 acres, and the total accretion at about 20 sq. m.; and the total area of inundated land north and south of the delta at nearly 60 sq. m.[1] He further estimated that the Po della Maestra advances 282 ft. annually, the Po delle Tolle 262 ft., the Po della Gnocca 11112 ft., and the Po di Goro 259 ft. The low ground between the lower Po and the lower Adige and the sea is known as Polesine, a name the derivation of which is much discussed. It is generally applied only to the province of Rovigo, but is sometimes extended to the neighbourhood of Adria and Ferrara. All along its course from Chivasso (below Turin) down to the delta the river is connected with several of its tributaries by canals, and at the same time other canals connect the tributaries and carry off their waters and the waters of the Po purely for purposes of irrigation.
The researches of Helbig (Die Italiker in der Po-Ebene, Leipzig, 1879) show that the lower valley of the Po was at an early period occupied by people of the Palaeolithic and Neolithic stages of civilization, who built houses on piles along the swampy borders of the streams. It is possible that even they may have begun by crude dikes the great system by which the waters are now controlled; at least it is certain that these works date their origin from pre-Roman antiquity. Pliny refers them to the Etruscans. The reclaiming and protecting of the riparian lands went on rapidly under the Romans, and in several places the rectangular divisions of the ground, still remarkably distinct, show the military character of some of the agricultural colonies. During the time of the barbarian invasions much of the protective system was allowed to fall into decay; but the latter part of the middle ages saw the works resumed with great energy, so that the main features of the present arrangement were in existence by the close of the 15th century.
The earlier Roman writers speak of the region between the northern boundaries of Etruria and Umbria and the Alps as Gallia Cisalpina. It was separate from Italy proper, the Aesis first and then the Rubicon being the boundary on the east, and the Arnus the boundary on the west, so that, for example, Luca remained outside the boundaries of Italy proper, even in 89 B.C. Romanization had, however, progressed considerable, the foundation of colonies and the construction of roads had, gone on during the 2nd century, and the whole district as far as the Padus was given the Roman franchise in 89 B.C., while the Transpadanes received Latin rights, and were fully enfranchised forty years later. Cisalpine Gaul was apparently formed into a province by Sulla in 81 B.C. and continued to be so until the fall of the Republic.
The Ligurian name of the Po was Bodincus or Bodencus, i.e. the bottomless. The name Padus was taken from the Celts or the Veneti. Thus we find Bodincomagus as a town name (Industria) on the upper course, and Παδόα (Padua, Catull. 95, 7) as a name of one of the mouths of the river. The name Ἠριδανός (Eridanus) of Greek poetry was identified with it at a comparatively late period.