The plain is the surface of an in-filled system of ancient canyons (the "Apennine Foredeep") 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 includes the Savio, Lamone and Reno to the south, the Adige, Brenta, Piave and Tagliamento 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 (Italian: Autorità di bacino del fiume Po), authorized in 1989 by Law no. 183/89 to oversee "protection of lands, water rehabilitation, the use and management of hydro resources for the rational 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-Romana 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 4 m (13 ft) below sea level in the Polesine subregion (the delta around Ferrara) to about 2,100 m (6,900 ft) 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 (draining Lake Maggiore), Adda (draining Lake Como), Oglio (draining Lake Iseo) and Mincio (draining Lake Garda and called Sarca in its upper reaches) in the north.

The Po Valley and the Adriatic overlay 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 (7-5 mya) 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, where much irreplaceable art and architecture is likely to be lost due to soaring sea level in the next centuries. Where the land surface now dips below sea level the river must run at a relative elevation between dikes.

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 (plain)". 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 ("spring") 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 50 km (31 mi) 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 (the Po and its affluents) 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 center of economic development and industry in Italy, and now is an almost continuous megalopolis strahing from Turin to Trieste.

The bassa Padana was settled and farmed earliest, in Etruscan and Roman times, and has been completely devoted 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.

The Po has a mild continental climate and a humid subtropical climate, depending on the part of the Po Valley one is referring to. Winters are not long, but foggy, damp and chilly, with sudden bursts of frost from the Siberian anticyclone; the urban heat effect has made them less foggy and cold than before. Snow was once commonplace, but is now decreased; prolonged winter droughts increasingly deny sufficient moisture to the soil. Spring and autumn are well marked and pleasant, while summer can be quite oppressive, hot and humid, with sudden violent hailstorms and, sometimes, what amounts to minor tornadoes. Both winter and summer are more extreme in the lower parts along the Po; the great lakes moderate the local climate around them. Wind is quite rare; only sudden bursts of foehn or thunderstorms manage to sweep the air clean. The almost enclosed nature of the Padan basin, indeed, added to the crushing weight of road traffic, makes it prone to a high level of pollution, especially in winter when cold air clings to the soil.