Basin of the po river 56? 



time probably a strait connecting the Adriatic with the Mediterranean through the 

 present Col d' Altare. The sea was gradually crowded out by the encroachment of 

 sediments, brought in from the Alps to the north and from the Apennines to the 

 south. Sediments from Alpine sources are coarse; from Apennine sources, fine. 

 The total area of the basin is 27,000 square miles, of which 16,000 square miles are 

 mountainous and 11,000 square miles belong to the plain of the Po. 



Borings in the plain show that it is composed of a series of approximately 

 horizontal sands, clays, and marsh deposits, the latter including lignitiferous clays. 

 The sands contain marine shells, the clays carry land shells. The whole succession 

 indicates alternation of marine, fresh water, and land conditions. The thickness 

 of the deposits ranges from 572 to 695 feet. 



There is little fine alluvium in the upper Po, the river flowing over coarse 

 deposits; but below the Sisera river alluvium of a fine type is common. 



The upper Po is everywhere crowded close to the northern spur of the Apennines, 

 forced over apparently by the large and heavily laden tributaries from the Alps. 

 In times of flood the river carries an immense amount of debris, estimated to be 

 3^o of its volume. In spite of this heavy load, the river is not aggrading its bed 

 to an appreciable extent. 



This non-aggradation is due in large measure to the lake system of 'northern 

 Italy, which drains into the Poand supplies it with T %ofits water content. During 

 periods of high water, in the fall and spring, the sediment-laden streams from the 

 Alps bring their load to the Po and deposit it. The lakes, however, being basins 

 of reception, not only take out the sediments from the drainage, but also store the 

 water and supply it more gradually than do the lakeless streams. Lago di Garda 

 in time of great rainfall scarcely changes its level ; the smaller lakes, such as Como 

 or Maggiore, show great changes of level within a few hours, but on the whole 

 they all tend to restrain the water. The result is that after the debris-laden streams 

 have deposited their sediments in the Po and temporarily raised its bed later there 

 comes a volume of comparatively clear water, which removes the previous accumu- 

 lations and an equilibrium is maintained on the whole. 



The Po is thoroughly diked from Cremona to the marshes of the delta. It is 

 customary to place the froldo or main dikes at some distance from the river, thus 

 allowing the river to overflow the intermediate flood plain or golene for some 

 distance before reaching the dikes. The golene are frequently covered with 

 willows and thick underbrush and the velocity of the current is greatly reduced 

 thereby and there is little active erosion upon the dike itself. The dikes are con- 

 tinually being extended ; the extension of dikes accounts in a measure for the rapid 

 extension of the delta in modern times. Between 1200 and 1600 A. D. the delta 

 advanced on the average about 70 feet annually; for the last few decades, its 

 advance has been at the rate of about 200 feet annually. 



The flood-plain deposits of the upper Po are cross-bedded and very irregular; 

 the beds are chiefly cobbles, coarse gravel, and pebbles ; occasionally wedges of 

 sand are thrust in, the latter of limited extent. 



The beds show great variations in size of materials; there are sudden changes 



from coarse to fine gravel and vice versa. The beds are not continuous over wide 



areas; generally there is a change in composition and texture every few rods. 



. Occasionally there are local deposits of silt and clay, stratified as a rule, which 



cover a few acres. One of these deposits in the environs of Turin covers 40 acres. 



On the lower Po the flood-plain deposits are much finer in texture and show 



