SEDIMENT OF THE PO. 273 



Lakes Maggiore and Como, and some smaller lacustrine reservoirs^ 

 and never reaches the sea. The Po is not continuously embanked 

 except for the lower half of its course. Above Cremona, there- 

 fore, it spreads and deposits sediment over a wide surface, and 

 the water vritlidrawn from it for irrigation at lower points, as 

 well as its inundations in the occasional ruptures of its banks, 

 carry over the adjacent soil a large amount of slime.* 



If to the estimated annual deposits of the Po at its mouth, we 

 add the earth and sand transported to the sea by the Adige, the 

 Brenta, and other less important streams, the prodigious mass of 

 detritus swept into Lago Maggiore by the Tosa, the Maggia, and 

 the Ticino, into the lake of Como by the Maira and the Adda, 

 into the lakes of Garda, Lugano, Iseo, and Idro, by their affluents,t 

 and the yet vaster heaps of pebbles, gravel and earth permanently 

 deposited by the torrents near their points of eruption from 

 mountain gorges, or spread over the wide plains of lower levels, 

 we may safely assume that we have an aggregate of not less than 

 ten times the quantity carried to the Adriatic by the Po, or 550,- 

 000,000 cubic yards of soHd matter, abstracted every year from 

 the Italian Alps and the Apennines, and removed out of their 

 domain by the force of running water. :}: 



* The quantity of sediment deposited by the Po on the plains which border 

 it, before the construction of the continuous dikes and in the floods which oc- 

 casionally burst through them, is vast, and the consequent elevation of those 

 plains is very considerable. I do not know that this latter point has been made 

 a subject of special investigation, but vineyards, with the vines still attached 

 to the elms which supported them, have been found two or three yards below 

 the present surface at various points on the plains of Lombardy. 



f The Po receives about four-tenths of its waters from these lakes. See 

 LoirBAKDOfT, Dei eangiamenti nelJa condizione del Po, p. 29. All the sediment 

 carried into the lakes by their tributaries is deposited in them, and the water 

 which floAvs out of them is perfectly limpid. From their proximity to the 

 Alps and the number of torrents which empty into them, they no doubt receive 

 vastly more transported matter than is contributed to the Po by the six-tenths 

 of its waters received from other sources. 



X Mengotti estimated the mass of solid matter annually "united to the wa- 

 ters of the Po" at 822,000,000 cubic metres, or nearly twenty times as much 

 as, according to Lombardini, that river delivers into the Adriatic. Castellani 

 supposes the computation of Mengotti to fall much below the truth, and there 

 can be no doubt that a vastly larger quantity of earth and gravel is washed 



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