The present rate of deposit at the mouth of the Po has continued since the year 1600, the previous advance of the coast, after the year 1200, having been only one third as rapid. The great increase of erosion and transport is ascribed by Lombardini chiefly to the destruction of the forests in the basin of that river and the valleys, of its tributaries, since the beginning of the seventeenth century.[231] We have no data to show the rate of deposit in any given century before the year 1200, and it doubtless varied according to the progress of population and the consequent extension of clearing and cultivation. The transporting power of torrents is greatest soon after their formation, because at that time their points of delivery are lower, and, of course, their general slope and velocity more rapid, than after years of erosion above, and deposit below, have depressed the beds of their mountain valleys, and elevated the channels of their lower course. Their eroding action also is most powerful at the same period, both because their mechanical force is then greatest, and because the loose earth and stones of freshly cleared forest ground are most easily removed. Many of the Alpine valleys west of the Ticino--that of the Dora Baltea for instance--were nearly stripped of their forests in the days of the Roman empire, others in the Middle Ages, and, of course, there must have been, at different periods before the year 1200, epochs when the erosion and transportation of solid matter from the Alps and the Apennines were as great as since the year 1600.
Upon the whole, we shall not greatly err if we assume that, for a period of not less than two thousand years, the walls of the basin of the Po--the Italian slope of the Alps, and the northern and northeastern declivities of the Apennines--have annually sent down into the Adriatic, the lakes, and the plains, not less than 150,000,000 cubic yards of earth and disintegrated rock. We have, then, an aggregate of 300,000,000,000 cubic yards of such material, which, allowing to the mountain surface in question an area of 50,000,000,000 square yards, would cover the whole to the depth of six yards.[232] There are very large portions of this area, where, as we know from ancient remains--roads, bridges, and the like--from other direct testimony, and from geological considerations, very little degradation has taken place within twenty centuries, and hence the quantity to be assigned to localities where the destructive causes have been most active is increased in proportion.