s in the
delta of the
Po river • The widest and largest rivers belong to the
Alpine region due to the arrangement and elevation of the relief, as well as the depth of the
Po Valley. • Along the
peninsula, given the arrangement of the
Apennine chain and the different slopes of the two sides, the rivers on the
Adriatic and
Ionian sides run through short transversal valleys and, except for the
Reno, do not exceed in length, while ten are just over . On the
Tyrrhenian side instead, they are on average longer because the Apennine spurs and the sub-Apennine belt are wider. • The rivers that flow into the Tyrrhenian sea are longer also because for the first stretch, they follow longitudinal valleys (Apennine valleys) and then run transversally with respect to the axis of the chain, in the south-Apennine area. • Given the location of the springs and the local rainfall regime, the rivers of Italy are divided into: • Alpine rivers, of glacial origin, subject to flooding in spring and summer because when it is hot the glaciers melt. The lakes that frequently occupy the most depressed parts of the Alpine valleys serve to dampen the rush of the rivers and to clarify their murky waters. In fact, given the rapidity of the valleys from which the rivers descend, the speed of their waters is remarkable and their activity of erosion and transport of rocky debris is sensitive. Decanting is precisely the process by which this material is abandoned in the lakes of which these rivers are tributaries. • Apennine rivers, subject to sudden spring and autumn floods due to the rains. The lean period is in summer accentuated in the Northern Apennines, almost absolute in the southern one, except for some waterways (
Aterno-Pescara,
Sele,
Volturno,
Liri-
Garigliano, to be limited to those that flow directly into the sea, to which are added
Velino,
Nera,
Aniene all in the , etc.) which are fed by large karst springs that spring on the edge of areas characterized by permeable fissured rocks. In fact, there are no snowfields and glaciers on the Apennines (the only glacier, albeit small, is that of the Calderone, on the northern side of
Corno Grande, in the
Gran Sasso d'Italia massif, in
Abruzzo); rainwater does not always collect in river beds made up of impermeable ground, that is, such as to allow a fair average annual flow. • The Sardinian and Sicilian rivers are torrential (full of water in winter and almost dry in summer), with the exception of
Tirso,
Flumendosa,
Coghinas and
Simeto. == List of rivers in Italy over ==