Just below the Great Salmon Falls the whole volume of the stream rushes through a channel hardly one hundred and fifty feet in width. At the falls themselves the river is nearly a mile across, and pours over a rocky wall stretching from shore to shore and about twenty feet high. It is fascinating in the extreme to watch the determined creatures as they shoot up the rapids with wonderful agility. They care neither for the seething torrent nor for the deep still pools, and with a rush--and with clenched teeth, perhaps--they dart up like a silver arrow, and defying rock and fall, are at length safe in the smooth haven above.
THE PO
GEORGE G. CHISHOLM
The northern plain of Italy, whose area is estimated at about 16,450 square miles, or about half that of Scotland, is a geographical unit of the most unmistakable kind. It is, indeed, made up of many river basins, but these are all of one character and without marked lines of delimitation. By far the greater part of the area belongs to the basin of the Po, and the rivers that do not belong to that basin present a general parallelism to the tributaries of the Po. The general slope of the plain is that indicated by the course of its main river, from west to east, but there is also a slope from north to south, and another from south to north, determining the general direction of at least the upper portions of the numerous affluents descending from the Alps and the Apennines. But before reaching the main stream, these affluents are affected in their general direction by the general easterly slope of the plain; that is to say, their course changes more or less to south-easterly (Dora Baltea, Sesia, Ticino, Adda, Oglio, Mincio), or north-easterly (Tanaro, Scrivia, Trebbia, Taro, Secchia, Panaro), and the farthest east they are the larger is the proportion of the entire course deflected in this manner. In the most easterly portion of the plain, lying west of the Adriatic, so marked is this effect that the rivers (Adige, Brenta, Piave, Livenza) are carried to the sea before reaching the Po. North of the Adriatic the slope and the general direction of the rivers (Tagliamento, Stella, Cormor) become wholly southerly.
Since ancient times the Po has been recognized as rising to the height of 6,400 feet in the marshy valley of Piano del Re at the foot of Monte Viso, the ancient Vesulus, and after a course of only twenty-one miles and a fall of 5,250 feet, it enters the plain at the bridge of Revello, where its middle course may be said to commence. Fed by the “aged snows” of the Alps, and by the heavy rains of the Alps and Apennines, it is already at Turin, where it receives from the west the Dora Riparia, a navigable stream with a width of 525 feet. At the mouth of the Ticino, the outlet of the Lago Maggiore, its lower course may be said to commence. Thence onwards it winds sluggishly across the great plains of
Fruitful Lombardy, The pleasant garden of great Italy,
with a mean depth of about six and one-half to fifteen and one-half feet, and a fall not exceeding 0.3:1,000, so that the waters could hardly move onwards were it not for the impetus imparted by the numerous mountain torrents which it receives at an acute angle. At last, charged thick with sediment, it passes onwards through the mouths that intersect its muddy delta into the Adriatic.
[Illustration: THE PO]
In this part of its course, artificial embankments have been found necessary to protect the surrounding country from inundation, and from Cremona onwards these dykes, in part of unknown antiquity, are continuous. After receiving the Mincio, the last tributary on the north, the Po assumes a south-easterly direction, which in ancient times and during the Middle Ages down to about 1150, it maintained to its mouths, passing Ferrara on the south, and then dividing into two main arms, the Po di Volano to the north, and the Po di Primaro to the south of the Valli di Comacchio. But about that date, it is said, the people of Ficarolo cut the dyke on the north side at Stellata, and thus gave rise to a new mouth, known first as the Po di Venezia, now as the Po della Maestra, by which the entire volume of the river now runs eastwards, till it breaks up into several small branches at the delta. Since then the arm of the Po between Stulata and Ferrara has become silted up. Since 1577 the Panaro which formerly entered this arm at Ferrara has gradually moved its mouth backwards till it enters the main stream just below Stellata. The Po di Volano, which in the Second Century B. C. was the most accessible mouth for shipping and afterwards the main mouth, has now become wholly detached from the Po, and merely serves as a drainage canal for the surrounding marshes, while the Po di Primaro has been utilized since 1770 as the mouth for the regulated Remo.
Long before the historic period, tens of thousands of years ago, but which geologists call recent, the great valley was an arm of the sea; for beneath the gravels and alluvia that form the soils of Piedmont and Lombardy, sea-shells of living species are found in well-known unconsolidated strata at no great depth. At this period the lakes of Como, Maggiore, and Garda may have been fiords, though much less deep than now. Later still, the Alpine valleys through which the affluents of the Po run were full to the brim with the huge old glaciers already referred to.
When we consider the vast size of the moraines shed from the ancient glaciers that fed the Po, it is evident that at all times, but especially during floods, vast havoc must often have occurred among the masses of loose _débris_. Stones, sand, and mud, rolled along the bottom and borne on in suspension, must have been scattered across the plains by the swollen waters.
It will thus be easily understood how the vast plains that bound the Po and its tributaries were gradually formed by the constant annual increase of river gravels and finer alluvia, and how these sediments rose in height by the overflow of the waters, and steadily encroached upon the sea by the growth of the delta. The fact that the drainage line of the plain lies not in the middle but farther from the Alps than the Apennines, shows that in this process the loftier range on the north has contributed more than the lower one to the south. And this process, begun thousands of years before history began, has largely altered the face of the country within historic times, and is powerfully in action at the present day.
It has been estimated by Sir Archibald Geikie that the area drained by the Po is on an average being lowered one foot in 729 years, and a corresponding amount of sediment carried away by the river.
It is hard to get at the historical records of the river more than two thousand years ago, though we may form a good guess as to its earlier geological history. Within the historical period extensive lakes and marshes (some of them probably old sea lagoons) lay within its plains, since gradually filled with sediment by periodical floods. The great lines of dykes that have been erected to guard against those floods have introduced an element that modifies this process. The result has been that the alluvial flats on either side of the river outside the dykes have long received but little addition of surface sediment, and their level is nearly stationary. It thus happens that most of the sediment that in old times would have been spread by overflows across the land is now hurried along towards the Adriatic, there, with the help of the Adige, steadily to advance the far-spreading alluvial flats that form the delta of the two rivers. But the confined river, unable by annual floods to dispose of part of its sediment, just as the dykes were increased in height, gradually raised its bottom by the deposition there of a portion of the transported material, so that the risk of occasional floods is again renewed. All these dangers have been increased by the wanton destruction of the forests of the Alps and Apennines, for when the shelter of the wood is gone, the heavy rains of summer easily wash the soil from the slopes down into the rivers, and many an upland pasture has by this process been turned into bare rock. In this way it happens that during the historical period the quantity of detritus borne onwards by the Po has much increased; and whereas between the years 1200 and 1600 the delta advanced on an average only about twenty-five yards a year, from 1600 to 1800 the annual advance has been more than seventy-five yards. Between 1823 and 1893 the deposits at the Po di Maestra and the Po di Goro advanced on an average 260 feet yearly, those of the Po di Tolle 315 feet, and those of the Po della Gnocca 110 feet. The area of the Po delta has increased within that time by twenty and one-half square miles, and that of the whole coast from 44° 20′ to the Austrian frontier by 29.8 square miles. Besides the Po and some of its chief tributaries, the Adige is the only river in the northern plain of Italy of importance as a waterway; and even it, though navigable for vessels of considerable size, as high as Trent in the Tirol, where there is a depth of from thirteen to sixteen feet, is navigable only with great difficulty in consequence of the great rapidity of its course. Boats can descend from Trent to Verona (fifty miles) in twenty-four hours, but for the ascent require from five to seven days. The country on the banks of this river is much subject to inundations, protection against which is afforded, as on the Po, by dykes, which begin about twelve miles below Verona.