SEDIMENT OF THE PO. 271 



"What proportion of the eartli with which they are charged 

 these rivers have borne out into deep water, during the last two 

 thousand years, we do not know ; but as they still transport enor- 

 mous quantities, as the North Adriatic appears to have shoaled 

 rapidly, and as long islands, composed in great part of fluviatile 

 deposits, have formed opposite their mouths, it must evidently 

 have been very great. The floods of the Po occur but once, or 

 sometimes twice, in a year.* At other times, its waters are com- 

 paratively limpid and seem to hold no great amount of mud or 

 fine sand in mechanical suspension ; but at high water it contains 

 a large proportion of solid matter, and, according to Lombardini, 



* In the earlier mediaeval centuries, when the declivities of the mountains 

 still retained a much larger proportion of their woods, the moderate annual 

 floods of the Po were occasioned by the melting of the snows on the lower 

 slopes, and, according to a passage of Tasso quoted by Castellani {DelV Influ- 

 enza delle Selve, i., p. 58, note), they took place in May. The usually more 

 violent inundations of later ages are due to rains, the waters of which are no 

 longer retained by a forest-soil, but conveyed at once to the rivers — and they 

 occur almost uniformly in the autumn or late summer. Castellani, on the 

 page just quoted, says that even so late as about 1780, the Po required a heavy 

 rain of a week to overflow its banks, but that forty years later it was some- 

 times raised to full flood in a single day. 



Pliny says : " The Po, which is inferior to no river in swiftness of current, 

 is in flood about the rising of the dog-star, the snow then melting, and though 

 so rapid in flow, it washes nothing from the soil, but leaves it increased in fer- 

 tility." — Natural History, Book iii., 20. 



The first terrible inundation of the Po in 1872 took place in May, and ap- 

 pears to have been occasioned by heavy rains on the southern flank of the 

 Alps, and to have received little accession from snow. The snow on the 

 higher Alps does not usually thaw so as to occasion floods before August, and 

 often considerably later. The more destructive flood of October, 1872, was 

 caused both by thaws in the high mountains and by an extraordinary fall of 

 rain. See River Embankments ; post. 



The irregularity of the precipitation in many parts of Italy is extreme. 

 Thus at Genoa, where the annual mean is 1285.98 millimetres, the rainfall in 

 October, 1833, was 16.18 millimetres ; in the same month in 1860, 18.15 milli- 

 metres ; in October, 1871, 24.62 millimetres, and in October, 1872, no less than 

 775.93 millimetres. Giornale di AgricoUura, 30 November, 1873. 



Pliny's remark as to enrichment of the soil by the floods appears to be veri- 

 fied in the case of that of October, 1872, for it was found that the water left 

 very extensively a thick deposit of slime on the fields. 



See a list of the historically known great inundations of the Po by the engi 

 neer Zuccholli in Torelli, Progetto di Legge per la Vendita di Bent incoUi 

 Eoma, 1872. 



