The Po 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 as “lower plain".
This specific meaning for "lower plain" derives from a geologic feature called "spring" line or zone, a strip of spring waters that varies from 2 to 30 km wide. This line is the outcrop, or intersection, of the anticline's water table with the surface at the edge of the “lower plain".
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 river waters meet very permeable soils that allow water flowing into the subsoil.
Then it meets an impermeable soil and it moves slowly from the top of the valley to the bottom. When the water meets a layer of clay, above another layer of clay, it is forced to rise near the “line of springs”. The water in these springs has got a temperature of 9-10°C in winter and 12-15°C in summer and it is rich of minerals. The water emerges in the “head” of the spring (the place where the water emerges) and then is distributed around it, where it can be used to irrigate fields.