sentences:  The Po collects all the waters provided within the semicircle of mountains (Alps and Apennines) which surround the region on three sides.From the highest peaks the land slopes down to hilly areas, (not always, though, sometimes there is a brusque transition from the mountains to the plains) and then to the upper, and then the lower Pianura Padana.The boundary between the first and the second is characterised by "risorgive", springs typical of the pianura padana which supply fresh water both to the rivers and to a dense network of irrigation canals.