Italy's Inland Waters
The characteristics of the Italian water network are closely associated with morphological and climatic conditions. There are only a few tens of watercourses longer than 100 km, though the Po, which is also the longest of them all (652 km) has a rainwater basin almost equal to a fourth of the national territory (74,970 sq km). Other important rivers are the Adige and Piave, descending from the Alps and flowing from the north into the Po, and the Arno and Tiber, flowing through central Italy into the Tyrrhenian. The other main tributaries of the Po are the Ticino, Adda and Oglio, arising in the Alps, the Tanaro, from the Apennines, and the Reno too, though it has its mouth to the south of the Po delta. The rivers running down the Tyrrhenian slopes of the peninsula are usually longe than those of the Adriatic, because of the Apennine watershed being further to the east. The Italian waterways are little used for transport due to their rather limited and variable flow. In fact the Alpine rivers have a cycle conditioned by the winter snow cover, being high in the summer and low in the winter; while the pre-Alpine and northern Apennine source rivers are mainly rain-fed and are only full in spring and autumn. Consequently, the cycle of the Po River is the most regular and therefore best suited to navigation. The other rivers of the peninsula and islands are heavily influenced by climatic conditions, being full in winter and empty in summer. In the latter case it is not unusual for the bed to remain completely dry, as in the case of the typical fiumare in Calabria and Sicily. Italy is fairly well supplied with lakes, having several thousand natural and artificial basins of different sizes and origins. The largest and deepest occupy the bottom of the great pre-Alpine valleys at their junction with the Po Plain (from Lake Orta to Lake Garda, which is the largest of all, while Lake Como is the deepest) and they were all excavated by Pleistocene glaciers. Also along the Apennine spine there are fairly frequent large lakes, such as Trasimeno the remains of an older lake that together with others occupied the bottom of the internal basins of the peninsula. The numerous small lakes scattered inside the spent craters of Latium and Campania are volcanic in origin. The coastal plains of the Tyrrhenian, Adriatic and large islands contain basins that are sometimes extensive and derived from lagoons. Furthermore, the Italian Alpine slopes, above 2,800 m., contain about a thousand glaciers. Some of these are of a considerable size, such as the Miage Glacier, which is some 10 km long and descends the southern slope of Mont Blanc in Valle d'Aosta The glaciers are especially important for their function as water reserves, providing as they do a constant supply for the Alpine rivers. The central Apennines also have a small glacier, under the northern walls of the Corno Grande (Gran Sasso). Finally, Italy's water system is completed by the many underground water bearing strata of the numerous limestone karst massifs in the pre-Alps and Apennines. These produce springs bearing a considerable volume (as that of the Peschiera in Latium or the Sele in Campania, etc.). In addition, there are those reaching to varying depths under the Po Plain and the other alluvial plains.